Why does Color Picker offer colors when I'm working in a Grayscale document?

[J Isner]

My document mode is Grayscale, but when I use the Color Picker, it offers me colors.  Specifically, When I do an Edit -> Fill and set K=20% (see below), the color actually added has unequal R, G, and B components (208,209,211).  After filling, if I check the color with the eyedropper, it reads 18%, not 20%.  Clearly, I misunderstand Grayscale mode.  What is the correct way to fill with a specific K value?

Photoshop 21.0.3 on Windows 10.

[Chuck Uebele]

Most likely too much trouble changing the color picker for grayscale and possibly other color spaces. I would keep my selections to the far left. That is the gray area, even in the color spaces. Also, PS might weight the colors from the color picker to give a more visual realistic rendering of colors. Because you're not actually picking a gray tone, from the left of the picker, PS might be interpreting that color. Like picking a CMYK color that's out of gamut. 

[J Isner]

But how can I use the color picker to choose a specific grayscale value?  When I entered K=20%, I got K=18%.

[D Fosse]

The numbers in the color picker relate to your working spaces - except for the open document, where the embedded profile overrides the working space.

But there is no grayscale readout in the color picker. A grayscale value is not the same as a CMYK K value. Grayscale is subject to color management just like any other mode. The numbers will vary according to your working gray. The same gray tone will give different numbers in Dot Gain 15% and in Dot Gain 25%.

Is this for printing on the black plate only? That's the usual scenario for working in grayscale. In that case you'll get better consistency by setting your working gray to <black ink> in your preferred CMYK profile:

You do this by clicking "load gray" in the rolldown, and picking the CMYK profile you want.

[J Isner]

Thank you, I learned something!  I'm trying to create a grayscale document containing a table of 101 grayscale values from 0% to 100% in steps of 1%.  When someone opens the document and samples the cells with the eyedropper, each cell must have the exact K value it was created with.  My Gray menu in Color Settings only offers the following choices (I'm using Gray Gamma 2.2 and Dot Gain 20%)

One way to do it (clumsy, but it works), is to fill the cell with black and then reduce opacity to the desired percentage.  For example, if Opacity = 30%, the eyedroper readout is K=30%.