Hello Acrylix! personally I use a layer in "color" mode on top of my b&w layer. For overlay and multiply, try using a less saturated color, like those you can find in the lower left quadrant when you open your color picker.
There are different methods and workflows, it's the artist who decides which one would be on the board. You can start with values and end up a colored illustration or start at once with it, just feel free to experiment which method works better to you, especially blending modes.
If you have any other questions just PM me, I'll be glad to help. Thanks for stopping by.
So do you work from black and white to color? I like to use that method because it lets me establish values first, but sometimes I have trouble adding the color over it. Especially browns, for some reason. Do you use overlay or multiply or a combination of whatever works?