I have had this happen too. I never quite got to the bottom of why the
palette reverted but it seemed to be when data or worksheets from
other files using the default palette were copied in. (But only
sometimes.) I got round it by keeping a file with the right colour
palette on my desktop and copying the palette over to the charts file.
this is in Tools/Options/Color in pre 2007 versions.

Chris B wrote:
I have set up an excel file with multiple graphs on one sheet. I seem to have
a re-occuring problem where the colours on the graphs change for no reason. I
think it is something to do with the pallet setup. For example on my pallet
today the bottom right colour is white whereas on Friday it was cream.
Subsequently any reference to this position in the pallet is affected by the
change.
Can you advise as this is causing me severe problems.