"Not enough colors"

Using xgdisplay in GRASS4.1 on a DEC5000, I sometimes get a message
XGRASS DISPLAY LIB ERROR: Not enough colors available. Make more
available before strating again.

At other times, xgdisplay starts just fine (and I love it!).

What is the source of this error? How do I make more colors available?
Any help appreciated.

Neel Smith
nsmith@polar.bowdoin.edu