This post is mostly aimed towards beginning builders, since much of this is common sense to someone with a few MOCs under their belt. Read this if you are just starting out building or if you feel like you need to bone up on what color scheme is and how to use it properly.
So let's get started. What is the first thing you should do when you start on a MOC? If you read the title, you probably already know the answer (It's color scheme) I can't stress how important it is. Often I have people who have seen my own models and want to start building themselves come out with a model that would be great if it had a unified color scheme.
A color scheme is the set of colors that you have in your MOC. There are some builders out there who attempt to make "Clowns" or models using completely random colors to give them a rainbow aesthetic. Building in whatever color you have on hand when you want to experiment with different building techniques, but I advise you never to make a finished MOC with more than 5 colors.
The image above is Stromius by Callan Lof. He's one of my favorite models* for reasons I'll get to in later posts, but one of the prime ones is the fact that there is a clear, consistent color scheme of translucent blue, white, and black throughout the entire model. It's a good color scheme in and of itself, but the best thing about it is that it doesn't deviate from it.
But wait! There is a translucent yellow light on the flash light and a grey pin in the waist. That's because it was essential for the flash light and no one is really going to notice for the grey pin. You see, just because you use a piece of one color it doesn't make it part of the color scheme. It would have been a problem if there were grey pins sticking out all over the model or if the trans yellow was used more than once. But if you use a piece once or twice, especially if its a necessity like a red axle or a blue axle-pin hybrid most people will forgive you.
Of course, not every color scheme works. I don't think it takes an experienced MOCer to know that a yellow, purple, and green figure just doesn't look good. It just takes someone that isn't color blind. However, sometimes people try to cram too many colors into a single MOC. Red, orange, grey, silver and black might sound good if you are choosing the color pallet for a two-hour movie, but it's probably too much to cram into a lego creation less than a foot tall.
Of course, piece shortage can be a problem when trying to build something with a consistent color scheme. The trick is to see what pieces you have the most of before actually building anything. Black and white are both awesome as colors for color schemes because they both are in plentiful supply and they both go well with most any other color. Black is especially good if you're just beginning to build because it is included in pretty much every Hero Factory set to date.
If you are serious about MOCing, you might want to strategically buy sets that have similar color schemes. Again, Hero Factory is particularly helpful in this regard because each 2013 set has a counter part with an identical color scheme. If you want to make a model with a yellow, blue, and black color scheme you would buy Evo, Aquagon, Surge and Bolt Dragon.
What about weapons? Should they follow the color scheme or go counter to it? Well, it depends. If you got into Bioncle from 2004 to 2008, then most of your weapons are probably silver, so using them on a non-silver MOC would be okay. On the other hand, its always nice to have a weapon that matches the rest of the body, especially if its something that is supposed to be organic like a claw or pincers. Take a look at these two uses of color-schemed weapons in creations by Toa Phosphorus:
Note that on the orange one (Entitled Toa Phosphorus) the weapons perfectly match the color scheme while on the black one (Kulstof) the gun is colored silver, a color not found anywhere else on the model. The thing is, either of these work, having a weapon that follows the color scheme and having one that doesn't. The important thing is that even if it doesn't follow the color scheme, it is still in a neutral color. If Kulstof had a blue gun, then it would have been a different story. Also, see how even though most of the gun doesn't correspond with the color scheme, there is still a trans-red piece on the top, hinting at the accent color visible elsewhere in the model.
Beginner: Make a MOC with only one or two colors, then when you have it the way you have it add another. See if its better before or after the addition of the new color.
Intermediate: Make a MOC using a rare color as an accent. Rare colors include teal, purple, dull gold and most translucent colors. Make sure to use colors in the right proportions.
Experienced: Make a team comprised of models with the same color scheme. Use weapons, size, proportion, and greeble use to make the models distinguishable from one another.
Boss challenge: Make a regular-sized MOC with only a single visible color.
Send in what you make!
*Everyone who I use a MOC as an example gave me their permission. If you have a MOC you want me to use, I'd be happy to use it as long as I can find a concept that it highlights. Also, I won't ever use someone else's MOC as a bad example of something. If I need to demonstrate how not to do something then I'll make something myself.