When I click on a node in the class browser tree widget, 
I get: 

TclError: unknown color name "darkblue".

This is coming from line 253 in TreeWidget.py where the 
background color is hard-coded as "darkblue".

On HP-UX 11.11 there is no entry for "darkblue" in 
rgb.txt.

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Given the apparent absence of problems on others systems, 
perhaps the absence of a 'darkblue' entry is a bug that should 
be reported to HP.  Do you know anything of the specs for 
rgb.txt?  And can you add such an entry to fix the problem?

If you are sure the absence if legit, what color can be 
depended on if 'darkblue' does not work?

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One of my collegues tried adding an entry to rgb.txt on
HP-UX and running the rgb util, but he couldn't get X to
recognise the new color.

Looking at the rgb.txt files on HP-UX and Linux (SuSE 8.2)
there  are many differences, not just color names that are
missing, but definitions which are different. 

I wrote a little script to compare the files. I have
attached its output.

However, the definition for 'darkblue' on Linux is identical
to 'blue4' on both Linux and HP-UX (0, 0, 139). 

Perhaps the simplest thing would be for IDLE to use 'blue4' ?

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Fixed by Nigel Rowe's Patch 
[ 805830 ] Highlight builtins