How find colors with certain value (blackness level) in Photoshop

Question

I'm learning digital painting in Photoshop, beginner level. I start with a composition in grey-scale, Let's say it is a mountain landscape in perspective with darkest object closest to viewer (a tree on a cliff) and lighter grey the further off into the horizon objects are (mountains).

Now I want to start applying color but preserve the values from the grey-scale sketch. I have a black & white adjustment layer on top of my drawing that I toggle to check that the values remain the same as I start applying color. Let's say I want the foreground to be green and red. How do I find greens and reds with the same value (amount of black) as in my grey scale concept drawing? Currently I just test out colors by handpicking them from a swatch palette and viewing them through the b/w adjustment layer, but there must be a more systematic way (I hope) to generate swatches from a value. Any advice or pointers in the right direction are highly appreciated.

(I'm using Photoshop CS4).

Answer 1

Simply painting on a layer above the greyscale image and setting the layer blend mode to "Color" will maintain the tonal value of the image while changing the color.

For example:

Another method is to simply paint the color you wish on a normal layer and then place your greyscale image above that layer and set the Greyscale layer blend mode to "Multiply" this will darken the color by the amount of grey in the original image.

Both methods have advantages. I encourage you to experiment to see what works best for you.