Skulls are about post count. The first thousand or so posts they grow from one to five, then glow and stuff, etc.
The name colors have to do with which "group" you're in. There are many different classifications of members on this board, each with it's own set of accesses.
For you normies, the color changes as you go up in post count. Under a thousand, you're orange because it's the default. If you join the PIT you become light blue. For those over two thousand, you turn (green? I think?), then over four thousand, (Yellow?).
Those going to Horror Realm are red (I think), as are Global Mods.
That's about what I know just from observing. Any other mysteries of the universe I can unravel for you?