Ok, here goes. I may not be the best of them out there (actually, I'm not even close

), but maybe some of my color advice may creep its way into your daily pixels.

Anyways, I like using RGB sliders, though much of the same rules about color selection according to Dhaos (Helm's is slightly different, but cool) apply here.
By the way, a lot of you may already know all this stuff in this section, so feel free to skip.

[COLOR SELECTION VIA RGB SLIDERS]
Anyways, I'm sure this is old-hat for much of you, but:
HUE: Is determined by the
general difference between the three values. By that, I mean "Is Red greater than green and blue? Is there more green here than blue?", Etc. etc.
Calculation of exact hue values is pretty complicated (I think, never tried to figure it out anyways... actually, now that I think about it, it isn't that daunting. Just a bit of your basic Trigonometry...).
Anyways, going on, some values of artificially pure colors are:
RED: RGB(255, 0, 0)
YELLOW: RGB(255, 255, 0)
GREEN: (0, 255, 0)
CYAN: (0, 255, 255)
BLUE: (0, 0, 255)
MAGENTA: (255, 0, 255)
These colors are at full saturation. To rotate through the color wheel at full saturation, you must have one color value at 255, one color value between 0 and 255 inclusive, and one at zero. E.G. this is at full saturation: RGB(25, 0, 255) (I'm pretty sure it's a bluish magenta or purple).
And FOR MOST PURPOSES, NO USAGE OF THESE COLORS (Now sometimes, it can be forgiven, but it has become somewhat of a pixel art taboo to use full saturation colors). This ties into the next part...
SATURATION: Ahh, so you ask, what colors am I suppose to use all the time??? Gray??? :s
Well, sorta, heh.
Anyways, saturation varies by, again, the difference between the color values. However, this time, you ask its
relative difference. Like, for example, "How close together are my color values".
The closer your RGB values are, the lower your saturation values will be. In the case of the extreme where the RGB values are equal, there will be NO saturation, meaning that this color will be black-gray-white or in between.
Examples:
A purty low saturation yeller: RGB(167, 167, 102)
Gray: RGB(127, 127, 127)
High saturation blurple (heh heh): RGB(245, 23, 128)
LUMINOSITY: ...or how bright it is. Question to ask here: "How much did I use?", and by that, I'm referring to color values. ALL colors at 100% luminosity will be PURE WHITE, and likewise, at 0% luminosity, will be PURE BLACK. Of course, translated to RGB terms:
RGB(255, 255, 255) You used the maximum amount of color values in that single color. Its at 100% lum. It's white.
RGB(0, 0, 0) ...You're smart enough to infer the significance of this I hope.
RGB(127, 127, 127) Teh colour is in teh middle brightness yay.
There is an important point that was mentioned by Dhaos, that I need to reinforce:
BECAUSE THIS IS AN ARTIFICIAL MEDIUM, (R + G + B) / (3 * 256) * 100 DOES NOT EQUAL LUMINOSITY.
Now you go, WTF ARE YOU TALKING ABOUT???
Well, besides the fact they don't use that formula to calc lum anyways (some weird crap about green), it needs to be pointed out that SOME HUES ARE BRIGHTER THAN OTHERS. Yes, your eyes are right, and not that lum slider (I hope, they might have taken that into account with the green *shrugs*).
Now tuck that into the back of your mind, because you'll ultimately know what colors are bright and what are dark when the time comes (unless you're blind. In that case... uh... WOW HOW DO YOU PIXEL???)
Anyways, going on from my little rant about RGB...
Anyways, for today's lesson... (ducks paper airplane) I'll be demonstrating colors through a nighttime city landscape (as opposed to a daylight natural landscape). As a side note, I always wanted to make one of these.
[STEP ONE - SELECTING JUMPING OFF COLORS - DEFINING SHAPES - REFILLING THAT CANISTER OF PURE CAFFIENE SITTING THERE ON YOUR DESK]

This is when you decide BG color, what tones to start out in, etc. I tend to start out with a nice midtone (some may use the darkest, and I'm pretty sure it's possible to start out with highlights).
As this is a nighttime cityscape, I chose a very dark blue background and somewhat lit midtones to block out buildings alternating between these to create different buildings. This is very important for establishing a basic mood (in this case, influence by the blues), and later color choices will bounce off these choices.
My midtones were relatively dark compared to what I could have used (excluding the moon). This also influenced by the darkness of the whole scene.
Note that purple is a "darker" hue than green, which I used to establish foregrounds, etc.
TOTAL COLORS AT THIS POINT: 4
Alrighty then... (ducks bullet).
[STEP TWO - SHADE - PURCHASE AN EXPENSIVE RENDERING PROGRAM - PLANT IT IN THE GROUND, WATER IT, AND WAIT TWO WEEKS FOR DELIVERY]

Define moar details w/ one or two darker shades of colors, re-using colors as you see fit. In this scenario, I decided to use a common darkest shade and reused the purple midtone for windows. I also created another grey for shading the moon (darkest shade reused there).
I added stars for purtyness.
TOTAL COLORS AT THIS POINT: 6
*Gasps* whew... (ducks Nuclear ICBM).
[STEP THREE - HIGHLIGHTS - BUY SOME SURPLUS HAIR PRODUCTS - COLOR YOUR HAIR GREEN - GO TO AN ANIME CONVENTION]

Hrrmm... highlights... reusing colors, I get that moonlight color to work for me by applying it in very small amounts. Then, I add a buffer to smooth the highlight out into each colour. The buffer is then used to highlight the purple building midground (a.k.a. purple, being more into background, gets the crap end of the stick

)
TOTAL COLORS AT THIS POINT: 7
OK! ... (ducks spacetime warp and tear machine of mass destruction... thingy).
[STEP FOUR - SECONDARY LIGHTSOURCES,
ANTIALIASING, AND COLOR REDUCTION - MY CONTACTS ARE FOGGING UP - IS THAT BLOOD???]

There is no reasoning behind my choice of a red secondary light source. That moon shade and the building color are too similar, so I combined them.
TOTAL COLORS AT FINIS: 7
Anyways, I conclude my short session about how I choose my colors. You may have noticed that I didn't unify my palette. This is due to partially:
a. My laziness
b. The mood of the piece is fine
I tend to adjust palette only to fine-tune atmosphere.
Besides that, Dhaos and Helm have fine examples of palette unification.
In any case, I may have restated most of the same crap you already know, but hopefully, you learned something from this. Ciao
