Use of libgd

[Doug Hardie]

I am trying to use libgd to create some images.  They are creating  
properly except there are color changes that I can't seem to figure  
out.  The initial image is basically black and white PNG.  I have  
converted it to GD2 format.  There are a number of secondary images  
that have a couple of colors (black, red, blue etc).  Each of them has  
been converted to GD2 format.  When any of these are converted back to  
PNG format they display properly.  However when I load the initial  
image, and then overlay it with one of the secondary images, the  
colors in the secondary images change.  For example, red can become  
yellow or black etc.  I tried gdImageCopyPalette from the secondary to  
the primary before the overlay but that seems to have no effect.   
Clearly I am missing something but can't seem to figure it out.  Any  
ideas?  Thanks.