Here are a few links I find useful in comparing RGB, HEX, and UCL or Munsell colors, though these don't all match the original Rock Color Chart, which is no longer available. The new Rock Color Chart colors do not match the old chart completely, and is organized in a way that I find difficult for real world use.
Geological ROCK-COLOR CHART - online pdf version (new)
The original - no longer available, though QCQA says, "Discontinued...Check with us later," and "Usually ships within 24 hours" — NOT.
RGB Chart & Multi Tool: HEX, RGB, Munsell.
Universal Color Language, Level 3: Munsell and HEX.
NBS/ISCC Color System: Munsell and HEX
Color Theory: Completed Munsell Color Charts - Gnomicon blog - 5R page.
http://colors.bravo9.com/nbs-iscc-rc-rock-color-chart/swatch/: This site is *still* down (permanently?), and my notes don't tell me what it was.
More About Colors:
Robert Krimen > Color-Library
The Michel-Lévy Interference Color Chart - although the colors look a little washed out