Well, it's certainly not the first time I've given tips on coloring. XDD I've been asked by a few random people... uh... Okay... Here's how I color images (most recent way):
I color by having manymany layers. D: One for each color. + the lineart and also one for the white background and another random color (usually grey).
Hair:
I do everything in Photoshop, btw. For hair I use the burn and dodge tools. Burn is always done first, and I make it so the hair has noticible shading (not TOO dark, mind). If they hair's REALLY dark (like Lyrus's was), then I make it so that the shading is near black, since half the time it's hard/impossible to see anyway.
Then I take the dodge tool and I always play around at first to see if I want to use highlights, midtones, or shadows for the tool (do this with the burn tool, too.) I usually use the setting that brings the color out most, which is usually Highlights. Sometimes it'll be Midtones, but it's rarely Shadows (which usually turns it white).
I use the dodge tool on all of the non-shadowed areas. Only use it enough so that there's some sort of bright color brought out, but not enough so that it becomes like... NEON colored. You'll understand if you try it.
Once that's done I take the brush tool with White as the color and set it to Color Dodge and the Flow to 1%. With that I go over the areas I want to have white shine (you can see on Lyrus's hair there's areas of blue highlights from the dodge tool and then patches of white shine to bring it all out). You learn through practice where to place the white, I think...
Skin + Clothing:
Uh... Skin and clothes (most fabrics) are done by locking the color layer's transparancy. Once that's done you use the eyedrop tool to sample the color and then slowly increase the darkness of it as you go over the areas you want to shade with the fuzzy brush tool (I usually use it at size 45-65, depending on the size of the image). Like... *grabs an image for help*

See how the colors get darker slowly? Well, the first one is the base skin color. Second is the first coating of shading, third is the second coating of shade, and so on. I keep going until it's dark enough for my tastes. This always varies with skin colors and clothing colors. Then once that's done, I take the dodge tool and set it to whichever of the settings makes the least difference, color-wise, and go over the center of the non-shaded areas with it.
Rando tips:
1: Remember your light sources! I HIGHLY doubt anyone here needs that tip, but eh, what the hey! XD
2: There's almost always shading on both sides of something. This is something I've noticed from personal experience. Unless the lightsource is EXACTLY on the side of something there will almost always be at LEAST a thin layer of shading on the side closest the light source.
3: More often than not, highlights are NOT white. Or anything near white. Except hair, where the white just serves to make it look really pretty.
4: Remember clothing folds! D: Folds ALWAYS have shading inside of them. And there are usually folds there that can't seem to be drawn in the lineart that should probably be included, too.
5: Hair has strands.
biggrin DD This is a good tip to always remember. It's not just a big blobby thing on top of the head. It varies in thickness, and for anime style hair, it takes a lot of shading. Like... Lyrus's hair does. o_O; His hair has a lot of peaks and falls.
6: Whatever you're coloring? It's a three dimensional object. I've seen this rule disregarded so many times it's not even funny. I tend to forget this rule when drawing the lineart, but I don't when coloring. What you're coloring has corners, curves, sides, whatever. Your highlighting and shading should take this into effect.
7: Use the brush side that fits your needs. This is good to remember because if you use a brush too large then it sometimes leaks over into other areas you don't want to have shading in. Too small a brush and everything takes forever to shade!
8: DO NOT USE THE BUCKET TOOL! I cant stress this enough. It ALWAYS leaves little thin white spaces between the lineart and the color, and it looks horrible. >__< I doubt anyone here does it, but I always include this tip when giving them.
If I think of more random crap, I'll tell you. XDDD