You could look at an old book called
Photo Oil Coloring for Fun & Profit by Lucile Robertson Marshall.
I found a copy on eBay for $8 once.
It's about painting on black and white prints to colorize them. You don't actually have to read it, but it may inspire you.
Richie Fahey sites the book as an inspiration. Take a look at his hand colored prints:
Is that the look you are after?