I use that method, what kind of issues are you having?

When I do that sort of coloring I usually paint a grayscale image, and then use a layer set to "color" on top of it. Only paint on that layer with colors that have maximum value, so that only hue and saturation get applied to the grayscale.

Works best for me, but the grayscale needs to be handling the values well, don't make everything in the same value range.

Thank you for response very much Artonis =) But thats not the way i use greyscale to be honest. There is a couple of things which are not working for me (and picky value work is one of them ^.^' )
At this moment I make base 3d shape rendering of the picture drawing with 50% of value as an underlayer. And using brush with about 25% (closer to black) (now experimenting with 11% and 35% too) above grey base using an overlay mode ( was trying also with multiply and hardlight thing). Mean I pick up the brush, and using pressure gradation of the values make the whole render of the picture (with limit of a darkness choosen by the blackness. like 25% as a limit of blackness) Still experimenting about adding or not adding highlights work too ( like 75% (closer to white). After greyscale in an overlay mode is done, I add flat colors udner it. And then with gradient maps I add some colors to an overlay render layer above the flats (helps to make colors more interesting and etc. )
But the problem I met, that sometimes shadows could be pushed too deep and with adding of color it will turn into black (if you push too dar colors on flats) and from the other side- if you push too light color on flats, the render layer (the one which is overlay) will start to decrease the contrast of the shadow work.
All ends up that I have a need to repeat shadow work after adding flats to an already 'rendered' picture. That was my problem. ANd I hoped if anyone can suggest something what will allow me to make most of the shadow work in greyscale and not to loose all the work after adding colors

when Ive worked gray to color something I found was the need to overpaint a bit after setting up the layer process if its a lot Id try copying the render and adjusting darks with curves to lessen the full black/ over dark areas

Hah. Thats somerhnig i came to recently =) Thank you for your comment here =)

I tried the Grisaille technique that I saw in this video.
https://youtu.be/nl99s-V3g5w
It has subtitles and is intuitive to practice. If the color doesn't suit your drawings, I recommend using gradation layers.

Try to use the multiply mode for the grayscale layer (lets the colors show through while preserving the shading) and adjust the opacity of the different colored layers for the visual effects.