Lets do this! How I add colour on my drawings and for this I take the help of a Dalek. Here’s the finished — it’s not finished as you’ll see later — drawing with just the values.
The next step is to add colours on a few layers set to the blend mode colour. This means that the values are left more or less untouched. I say more or less as some colours such as yellow is problematic and might not work as intended straight out of the box with some values. Anyways, I use a few layers since this means I can paint over and under the previous colours.
See? Depending on details, I think I used three colour layers so far. Then I add an overlay layer, in this case too add some coloured tint and shadows — had I done this with a multiply layer I would have had to work a whole lot more with the opacity.
And then: soft light to add some highlights.
Adding details on a normal layer now. Since we now have the colours defined this is easy.
As a very small finishing touch a colour layer to give a blue reflection.
I usually do some post-colour tweaking with gradient maps and colour balance adjustment layers as well as a photo-grain filter to fuzzy it up a bit, but neither of those are pictured here.
Now. Go out there and ex-termin-ate while having some cookies.