What sort of colors are appropriate to Steampunk? The simple answer is: all of
them. Even without the science fiction aspect of Steampunk, Victorian fashion
could actually be quite colorful, particularly for women. They might have been
prudish, but they weren't Puritans; they were in no way limited to black and
white (although the fact that Victorian photography was black and white might
be one reason why people think they were.)
Bright, saturated colors help to imply wealth for two reasons: strong dyes were likely to cost more, and they give the appearance of cleanliness.
More subdued, muted, less saturated colors (that is to say, colors that have been mixed with grey or brown) give the impression of a more practical, working-class garment.
In steampunk, brown is the new black. In part, this is because leather is most commonly brown, and leather is a common material for steampunk costumes. Leather is a tough, practical and protective fabric while still being attractive. (To be fair, most of the leather actually used in steampunk costumes is not particularly tough or practical, but this is fantasy, after all.) And, let's face it: we like leather, and leather is in right now.
But there's also some practical value to neutral colors: they mix-and-match very well. If you're someone who creates complete outfits, this isn't much of a concern for you: you just make sure all the colors involved complement each other. But if you're like me and pick up a piece at a time, regularly switching different elements in and out, neutrals are a godsend.
Many of my outfits feature one colored item paired up with other items in a variety of neutrals. My bright red corset works great with black or white skirts, but not my yellow skirt. The yellow skirt goes well with my brown leather corsets, grey and black overskirts, and black, white, or beige shirts. This system allows me to have a wide variety of outfits while owning a more limited number of garments; seven outfits doesn't require seven shirts, seven corsets, seven underskirts and seven overskirts, which would leave me broke.