Ok, it’s a very easy process actually. I’ve made a rough example for you ;)
I want to color a Billie b/w image: first thing I need to do is load both the b/w image and a non b/w image of the same photo-shoot or at least a colored photo of the same actress. Why? Because we need to take all the colors-samples and keep them as real as we can.
Let’s start with the hair:
Pick the most dark shade of her hair and with a brush* color the hair on the b/w pic [set the brush to softlight]
This is the first result: as you can see the hair is too light and it doesn’t look very realistic. So our next step is to give a much more warmer effect, therefore I’ve added an orange-y color and with a soft brush -still set to softlight - I’ve colored the hair for a second time.
It’s already better, but it’s important to pay attention to the shadows of the hair/locks, so I apply the last softlight brushing and this time it’s plain black.
*The brush is one option, but with the Lasso tool you can select a wider area and color quickly. Personally I prefer the brush: it does the job better.
Hope it was clear enough! :)
[Just keep in mind that the softlight blending doesn’t always work. For the face for example I sometimes use overlay or hardlight or even linear burn. So keep playing around with the different blending options (and the different shades of color) to get the nicer result :)]