So, what's the best, most accurate way to do a grayscale check in Photoshop?

Question

In other words, as an artist who sometimes uses Photoshop to draw and paint, I like checking if my values are working so I have the practice of switching my work back and forth between grayscale and srgb, but as you might know there are multiple ways to do this, such as:

Adding a "Black & White" adjustment layer and bringing it to the top.
Adding a "Color Fill" adjustment layer with black as the contents, changing the blend mode to "Color" and bringing it to the top.
Adding a "Hue/Saturation" adjustment layer, adjusting the "Saturation" slider all the way to the left (-100) and bringing it to the top. (This one is for sure the most innacurate and no one should do this but I'm putting it here anyways)
View > Proof setup > Custom... > Dot Gain 20%. (I saw this one in a YouTube video about colours by the talented Marco Bucci)

So, it would be very helpful to know which one is the most correct method to check values in grayscale when working in Photoshop. Please do tell me if I missed another one that would be more accurate.

I ran some tests, and every single way differs in some way from the other, with none of them being exactly the same when tested over images that are already, by themselves, grayscale.

Stephen Marsh · Accepted Answer

Adding a "Color Fill" adjustment layer with black as the contents, changing the blend mode to "Color" and bringing it to the top.By @grunge616  This creates the true RGB Luminosity of the image (the RGB equivalent of the Lightness of Lab colour mode), so I'd go with that. Filling with black, white or any gray value should achieve the same result when set to color blend mode).

D Fosse · Answer

The thing about grayscale is that there's always a tone response curve. So there really is no absolute reference grayscale.

In addition, desaturating from RGB can be done in several ways. It can either take the inherent brightness of colors into consideration, as Lab does, or it can ignore that and simply average out the RGB numbers. The difference is dramatic:

Photoshop's Luminosity mode is a hybrid. It is based on the RGB numbers, but weighs them differently according to how sensitive the eye is to different wavelengths. The final formula is somewhere around 25-60-15 or thereabouts (can't recall the exact numbers). Luminosity is similar to Lab L, but it's not identical.

If the emphasis here is on reference, I'd go with Lab L. Lab is at the heart of all color management as a profile connection space (PCS).

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