Alright, so, a long time ago I made a pretty crappy tutorial about value sketching characters and environments that, looking back on, I'm not too proud of. It was confusing, didn't really explore a lot of important concepts and was far too rushed. So I'm revisiting that tutorial, except I'm taking it slow and breaking it down much further. So for those of you who read my Concept Art 101 tutorials, consider this an upgrade.
Uhm, alright, so, to get started: value. Plenty of us on MT have experience with shading and coloring our pictures, beit purely values or under some lineart. Now, we usually use the term "value sketch" to refer to a lineless drawing shaded completely in black and white. This is actually a very deceptive term because value is present in everything we do, even lineart. It just doesn't matter so much in lineart. So what does "value" really mean, then?
Value is the perceived lightness or darkness of a color or tone. For example, black is the absolute lowest value you can have. You can't get darker than black. Oppositely, white is the lightest value you can have. You can't get lighter than white. The value of every other tone and color fits on a gradient between black and white.
Now why are values important? Value is what gives depth and form to a picture. Having a good range of values in a colored or shaded piece will make it look fuller and more three dimensional. It's all about contrast. Now, that's not to say you can't get away with coloring with the same value. Colors are diverse enough that you can probably make out shapes based on the differences in color. For example:
Here is a picture of a circle and a random polygon (a cube, but we'll save that distinction for later). Believe it or not, these two colors share the same value. As proof, I've also included a desaturated version. Now, you might look at this and say: "well, I can tell them apart quite easily, so why does value matter?"
I'll show you why:
Here, I've "shaded" the shapes with the opposite color. I've tried my best to turn the circle into a sphere and the polygon into a cube...
You're probably looking at it like, "wow, that doesn't look shaded at all. It just looks like you colored two flat shapes two different colors." And why is that? Because the values for the "light" areas versus the "shadow" areas are the same. The lack of difference in value causes our eyes not to perceive one color as shadow and one color as light. Instead, they just look like two flat colors.
This is why value is important because, when you're actually trying to create depth and form with your shading or coloring, you don't want your final product to look "flat". You want a large range of values that will make your final product "pop". Sometimes when people are coloring, they're like, "hey, why are my colors getting muddy? Why doesn't my shading look 3D?" This is your answer. Your value range is either too narrow or nonexistent (ie. the colors share the same values). Even if colors may appear different, if they share the same value, all you end up doing is mixing flat colors.
Now, I'm not going to teach you how to translate color to value. That just comes with experience and training your eyes. But one rule of thumb worth remembering is that darker colors = darker values. Programs like Photoshop and GIMP make this easier by including a separate value slider or indicator. It's also a good habit to, every once in a while, desaturate your drawing and see it in black & white. This will help you make sure the values are well separated.