If I intend to colour it, I plan the colours as I'm drawing, so that the balance is right between the colours of the image. You might reaaaallly want a green outfit but if the outfit is just one huge block of green it will look a little weird, so you have to plan the balance so you don't just get blocks of colour, which you might get if you just did a lineart sketch with no colours in mind. That's not to say you can't go "oh this looks nice I'll colour it" but it's also good to plan as you go.
I quite like to sketch with blocks of shade so I know which areas will be darker as this also effects composition.
Looking at other pieces and photos is easily the best way to start incorporating color into your work. Color theory is great but the application of it to your own piece can leave gaps in how to handle say an orange light bouncing off a colored surface surface onto skin. Seeing the way other artists have handled the colors and transitions into other colors on a form can teach you a lot, and bring you to the point where the stuff you pull out of your head will be as colorful as you want while still retaining their accuracy. As with drawing, studying from life is the best way to pull stuff out of your ass later on.
2. Pick a couple of other colors that look good next to it. Ideally they should have different values and saturation, so there is contrast going on there, as well as in hue.
3. Draw shapes. For shading, I'll usually just add one of the colors in multiply mode. If I'm being super-graphic I'll just have a sharp transition to one of the darker colors in my palette.
4. If I need a new color for some detail, then pick it too.
5. At any point in this process, I might tweak the colors. I work in Illustrator, so I have this magic ability to make everything drawn in one swatch change when I change the swatch. Sometimes there's a dramatic color change as one of the very last things I do to a drawing!
Sometimes I'll start with a quick messy color rough under my sketch, other times I just dive in and start making shapes, and tweak the colors as I go. Sometimes I'll limit myself to just two or three swatches; this results in a very unified palette.
I do a rough concept sketch of where stuff is going to be placed in the image, and then I just slap down colours until I end up with something that I like. I usually try to figure out what kind of "atmosphere" I want the piece to have (so like, warm versus cool colours), and I've been trying to pay attention to things like formal colour theory but it's mostly just trial and error.
I'm not super comfortable with colours so I tend to do very limited palettes, usually with more narrow areas of the colour wheel, and I use a lot of neutrals to balance things. Also I really like blues and golds. A lot.