The encoding scheme used uses 1 8-bit byte. That byte uses 3 bits for red and 3 bits for green, but only 2 bits for blue (it's referred to int the VNC/RFB spec as BGR233). That gives seven discrete values for red and green, but only four for blue. There is some vestigial support for colormaps, but I'm pretty sure it's never been completed. I could use 24-bit colors, but that seems like a lot of overkill(including network bandwidth) for a 2-bit display (no pun intended).

If you care, the levels are:

255 219 182 146 109 73 36 0

255 222 181 148 107 74 33 0

255 173 82 0

for green, red, and blue, respectively, on a 0-255 scale. At least that's how I read it from WinVNC client->Win screenshot->clipboard->paste->Photoshop. Seems to match up on other clients that way, too. I don't really understand the discrepancy between red and green, but there you go.