Edit 2
Today's
[Image: http://puu.sh/evuW]
These are relatively random colors but they can be made to work together. For now all that matters is the way I do things, the object I'm shading is separate from the background (or other layers with other colors on them). I also lock the transparency of the layer once I've got the base color down, because there's no need for unnecessary headaches.
[Image: http://puu.sh/evw2]
I tend to work with brushes that hover between the 15-30% opacity area, so the colors that show up as shadows appear significantly lighter than what I chose. This way, I can layer the brush strokes on top of each-other and control how dark or light it gets.
[Image: http://puu.sh/evwN]
This is after a few brush strokes. I didn't change the settings or the color. The dark purple around the middle is what the color I picked actually looks like, but that's not always dark enough, nor is using a single color for a shadow a good idea. I chose not just a darker shade, but a slightly different color. Going towards blues is generally safe as a starting point.
[Image: http://puu.sh/evxB]
Some extra shadow. It's exaggerated but it's mainly to get a point. You can fix something that's too dark by eye-dropping from a color around it. The easy way to do that is to just keep your brush as your tool and just hold down the alt key and click, and that will automatically eye-drop. Really handy for when you're needing to blend things into each-other and are going to be needing different colors for different brush strokes.
Before blending I'll actually get into the highlights a bit.
[Image: http://puu.sh/evyy]
Yellows are a safe route, because they're warm and pop against the blue shadows. As long as you blend properly, it can work on anything. I use it in basically everything to sort of unify the whole piece, but the further away from the light source, the less yellow it should look; the less it should pop.
The highlight was done the same way as the shadow. Just pick a color that's close to the "max" highlight I want, then use multiple brush strokes to get it to show up lighter. The pinkish-orange color is closer to what my brush color actually is.
For the actual blending I'll use this..
Starting point for blending should be something like:
[Image: http://puu.sh/evA3]
To blend, just stay on your brush tool, hold down the alt key, and click on one of the purples in the middle, then let go of the alt key, and your color should be what you just clicked on. With a low opacity brush (it should be low already anyway) make a stroke in the middle. Repeat the same brush motion a few times and it should look like this:
[Image: http://puu.sh/evAE]
You'll notice that on either side of the strokes you just made there's now a distinct line. What you do at this point is basically repeat what you just did. Grab a color in between the light and mid-purple and make a few strokes over that distinct line, then grab a color between that mid and dark purple and make a few strokes there. How far you want to take this blending is really up to you. For me it depends on the kind of shadow it is, since there are some really sharp shadows that work best with less blending.
[Image: http://puu.sh/evC4]
You can easily go from a harsh shadow to a soft shadow by only blending part of it, or even intentionally grabbing a darker color and bringing it in further into the light color.
For blending, the real thing to remember is the alt key is your friend.
I'll go back to the badly-lit "sphere" and blend it myself.
Remember it's important to follow the shape of whatever object you're shading with your strokes, or it could start looking distorted.
[Image: http://puu.sh/evDv]
So something like that. It's not too great but it starts to get the point across. All of that is done with one base color, one highlight color, and 2 shadow colors max, so you could really take this a lot further. I won't really bother with that for now, except for one thing: It's always important to take the colors surrounding the object into account. Another one of those things that unifies the piece.
Blending the color in is pretty easy. Just alt+click like before, and make a brush stroke near the edge of the object with a low opacity brush. If you're feeling up to it you can do more than just one, but this color needs to be a bit more subtle than the others.
[Image: http://puu.sh/evEs]
Something like that could be enough as a starting point, then just blend from there.
[Image: http://puu.sh/evEV]
The end result could be something like that, perhaps? And remember you can always tone down something you think you went overboard just by using the immediate surrounding colors.
This probably would have been better off as a tutorial with better screenshots and me pointing out things more specifically with my own brush, but that's a bit too much work for a doodle and I'm still kind of lazy as a person in general....
:whoops:
EDIT;
Sidenote: I work in large-resolution images (at least 300 DPI each) and it's probably best to get into that habit if you're going to keep doing this. Once you zoom out of things, they look even more blended. 8D
Otto, when I look at my old stuff I don't know if I want to laugh or cry.. XD