You can use 26 named colors from the UI of IPE. Sometimes, you do need to other colors for your diagrams. For example, some basic colors are missing like cyan (the combination of green and blue).
Thankfully, the full range of named colors typically used in HTML is available. A whopping 657 of these named colors are in the
colors.isy stylesheet, which is located in
/usr/share/ipe/7.1.4/styles. To get these colors into the color menu in IPE, just load this stylesheet as described here.
Tried with: IPE 7.1.4 and Ubuntu 14.04