don't worry sherlock medic's on the case
normally i try to stick with warm color schemes
and when it comes to tf2 art, i don't use the palette valve gave for the colors because they're not bright enough. even cool colors like blue and green would have a sorta warm feel to them. if the colors aren't warm enough to my liking, i adjust them using photoshop.
either image => adjustments => hue/saturation or
image => adjustments => photo filters work well
i also don't use black. They're normally very dark shades of other colors. All grays also have a warmish red tinge to them (of course there are ways to use black properly, but i tend to not to unless im lazy/experimenting)
and there are also times i take color schemes from things i've seen like art, photographs, etc.
it's basically a jumble of warm colors, bright colors, colors i've seen before, and photoshop enhancing. hope this was at least a little bit helpful!