Blue Screen Test

Press Go full screen and the display turns pure blue, ready for a pixel or night-mode check.

Colour Pick one, then go full screen.

What to look for

F goes full screen. Escape leaves.

Pure blue

Pure blue

#0000FF · 7.2 percent of full light

#0000FF. Red and green stay dark and blue works alone. Blue adds the least to brightness, so the whole field gives only about 7.2 percent of what a white screen gives.

The page's own dimmer, not your device's brightness.

What to look for

Checking the screen on blue

Checking a night mode or blue-light filter

As a blue backdrop

Next step Next: flat grays and colors for uneven patches

How it works

This blue screen test shows a blue color full screen in one press: pure blue, #0000FF. On it a faulty sub-pixel shows as a dot, and a night mode or blue-light filter shows as a darker field.

Read the full guide

Check the screen, then the filter

1. Press Go full screen on pure blue. 2. Look for dots that are not blue. The notes below name each color. 3. Switch your night mode on. Pure blue should turn darker; if nothing changes, the filter is not active.

Each blue screen color and its code

Pure blue, medium blue and navy blue light the blue sub-pixels alone, at levels 255, 205 and 128. The other five mix in green and red, up to light blue. Each prints its hex code and its share of white's light. Copy hex copies the code, ready to paste as a blue screen background in a slide or an editing program.

  • Raise the device's brightness first: pure blue gives about 7 percent of white's light.
  • Keys 1 to 8 change the blue while the page is full screen.
  • No shade is named neon blue: for a neon blue screen, try pure blue or deep sky blue.

Questions and answers

How do I test a blue screen on my monitor?

Press Go full screen for a full screen blue and look across it for dots and uneven patches. This is a plain blue color field, not the error screen a computer shows when it crashes. No web page can produce that one.

Is there a light blue screen or a bright blue screen here?

Both. Light blue, #ADD8E6, is the palest shade, at about 64 percent of white's light. For a bright blue screen that still reads as blue, pick dodger blue or deep sky blue. A pure blue screen gives only about 7 percent.

What is a blue light screen test?

It shows whether a night mode or blue-light filter is active. Put pure blue on screen and switch the filter on: the field darkens, because the filter cuts blue. This page shows the change by eye and says nothing about eyes or sleep.

What does a dot on a blue screen mean?

A black dot is a blue sub-pixel that stays off. Magenta means a red sub-pixel is stuck on, cyan a green one, and white both. Blue is dim, so look closely.

Why does pure blue look so dark?

Blue adds little to brightness. Pure blue gives about 7 percent of the light of pure white, against about 72 percent for pure green. A dim blue field is normal and not a fault.

All 17 shades and checks

Blues, deep to pale 8

  • Pure blue #0000FF. Red and green stay dark and blue works alone. Blue adds the least to brightness, so the whole field gives only about 7.2 percent of what a white screen gives.
  • Medium blue #0000CD: blue alone at level 205 of 255, about 4.4 percent of white's light. Stepping between it and pure blue shows whether your screen still separates the top of the blue channel.
  • Navy blue #000080, which CSS names navy: blue alone at level 128 of 255. Half the signal gives about 1.6 percent of white's light, close to black in a lit room and plainly blue in a dark one.
  • Royal blue #4169E1: a strong mid blue with some red and green mixed in, at about 17 percent of white's light. Brighter than pure blue, because the green sub-pixels are lit.
  • Dodger blue #1E90FF: a bright, clean blue with green above half, at about 27 percent of white's light. A lively backdrop color that stays clearly blue on a dim screen.
Show all 17 shades and checks (12 more)

Blues, deep to pale, continued

  • Deep sky blue #00BFFF: blue at full with green at three quarters and no red, about 44 percent of white's light. On this shade a red sub-pixel stuck on shows as a pale pink point.
  • Sky blue #87CEEB: a soft daytime blue with all three sub-pixels lit, at about 55 percent of white's light. A calm wash for a call or a slide.
  • Light blue #ADD8E6: the palest blue here, at about 64 percent of white's light. It carries plenty of red and green, so a night mode shifts its tint instead of darkening it.

Checking the screen on blue 3

  • Dots on blue and what each color means Black: a blue sub-pixel that stays off. Magenta: a red one stuck on, adding red to the blue. Cyan: a green one stuck on. White: both. Blue is dim, so raise the device's brightness and look closely.
  • Navy blue that looks black Navy blue gives about 1.6 percent of white's light. Open the black screen in a second window beside it: navy should still look blue. If the two look the same, the darkest blue levels are being lost.
  • Why pure blue looks so dark Pure blue gives about 7 percent of the light of pure white, pure red about 21 and pure green about 72. A dim blue field is how screens are built and not a fault. Navy blue is dimmer still.

Checking a night mode or blue-light filter 4

  • Pure blue darkens when a night mode is on A night mode shows warmer colors by cutting the blue light of the display. Pure blue is blue light and nothing else, so with the mode on the field turns visibly darker. Switch it on and off with this shade up to watch.
  • Seeing how strong the filter is Raise the strength of the night mode one step at a time with pure blue on show. The darker the field gets, the more blue that setting removes. The page shows the change by eye and cannot put a number on it.
  • Pale blues shift instead of darkening Light blue and sky blue carry red and green as well as blue. A night mode takes some of the blue away and leaves the rest, so they drift toward gray or green while staying bright.
  • A filter left on by mistake If pure blue looks dull at midday, look for a night mode on a schedule or a warm color temperature in the monitor's own menu. Either one lowers blue across the whole display, this page included.

As a blue backdrop 2

  • Blue for keying a green subject A key removes everything near the backdrop color. Film makers use a blue backdrop when the subject is mostly green, such as a plant, which a green one would erase. The same holds for a small object in front of this screen.
  • A plain blue slide on a projector Full screen on a projector gives a plain blue field between items: far dimmer than a white slide, with no desktop on show. The share link reopens the same blue.

How this list was compiled: Shades, checks and uses chosen and written for this page. Every hex value is a CSS named color, taken from the table of named colors at https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/named-color, read 2026-10-02. The share of light printed for each shade is its relative luminance (0.2126 R + 0.7152 G + 0.0722 B on linear values, pure white as 100), worked out from the formulas at https://en.wikipedia.org/wiki/SRGB, read 2026-10-02. That a pixel is made of three sub-pixels, that a dark dot is one that stays off and a stuck sub-pixel one that stays lit, is from https://en.wikipedia.org/wiki/Defective_pixel; that red and green light make yellow, red and blue magenta, green and blue cyan, and all three white, is from https://en.wikipedia.org/wiki/RGB_color_model; both read 2026-10-02. That a night mode shows warmer colors by reducing the blue light of the display is from https://en.wikipedia.org/wiki/Night_Shift_(software) and https://support.microsoft.com/en-us/windows/set-your-display-for-night-time-in-windows-18fe903a-e0a1-8326-4c68-fd23d7aaf136, read 2026-10-02. A blue backdrop for a subject that is mostly green is from https://en.wikipedia.org/wiki/Chroma_key, read 2026-10-02. A color on a screen is not a match for paint, print or another screen, and nothing here is a measurement. Licence: own work

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