In Python 2.7.9 for Windows, colors displayed match their RGB values as defined in TclTk:
http://www.tcl.tk/man/tcl8.5/TkCmd/colors.htm (8.6 is identical)

In Python 3.4.3 for Windows, the following colors differ noticeably from their TclTk counterparts: grey/gray, green, purple, and maroon.

Instead of the spec TclTk RGB values, these particular colors are rendered using the HTML RGB values.

This only happens in Python for Windows - OSX doesn't have this problem as it appears to use the same TclTk package for both 2.7 and 3.4 and correctly renders the colors per TclTk specs in each.

Tkinter ought to render named colors using the spec TclTk RGB values regardless of platform.