: Why is pure blue darker than pure green? If I convert 100% green and blue to grayscale, the blue one becomes about twice as dark. I can see that green looks brighter, but I also want to

@Angela777

Its because of the function used internally when converting to grayscale. Its a weighted ratio addition of the three components, with more weight assigned to green compared to blue.

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@Eichhorn212

The best way to inspect the differences in lightness is to use the Lab colour space, rather than RGB or CMYK.

The Lab colour space was developed to be a better representation of human vision than the RGB colour space. The L channel in particular, which stands for "lightness" and is the relevant channel for this discussion, attempts to be a closer match of human perception of lightness.

If you compare a pure green vs pure blue image, you'll see that the lightness channel has different values. While pure blue has a value of 30 (out of 100), green has a value of 88.

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@Phylliss782

I find that's it's easier to evaluate RGB colors in the HSB model (Hue, Saturation, Brightness). In this case, it helps you in two ways.


1. It helps me remember that some colors have greater inherent luminance/brightness than others. Just take a look at the green and blue points on the Hue scale. Green is clearly brighter.


2. If you're looking to control the value levels, you have a clear indication in the brightness scale (once you remove the saturation variable).

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@Nimeshi706

"Greyscale" is a print specification basically. Yes it removes color, but the "Greyscale" mode is only really needed for printing. Everything on screen is RGB even if it looks grey. In this respect, when you use the Mode > Greyscale command, Photoshop ignores the RGB data and relies on CMYK color data and your color profile settings to convert.

If you hover your cursor over the colors and look at the Info Panel you'll see the wildly different CMYK colors:





You can see that one color is 69C and 100Y and the other is 90C and 72M (based on my profile settings). Mixing C and Y is always going to be lighter than mixing C and M.
(also note the ! next to the values indicating that the color is actually out of the CMYk gamut.)

This is a simplified explanation, as others have pointed out luminosity is a large factor as well in the conversion.

If you are strictly working with RGB documents, greyscale is a poor choice. You are better served with a Hue and Saturation Adjustment layer which simply removes all saturation -- this results in the same grey for both colors if that's what you want.

You can also use Image > Adjustments > Black & White to convert to a grey image (still in RGB color space though). This allows you to tweak the levels of the colors specifically to achieve the greys you want.

Essentially you're comparing RGB and Greyscale when they really don't have any correlation.

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