strange artifact in two-color images (Intel Mac)

I am trying to draw a simple two-color image (e.g. any pixel in the
image is either red or blue, and no other color appears in the
image). The image is a 2-D plane with each pixel pointing to a row in
a 256x3 color table. I put my two colors in the first and last row of
the color table, and every pixel in the image is either 0 or 255.
When the image is displayed ('DrawTexture') there is a line drawn in
a third color (black or white, depending on the color map that I
started with) anywhere there is a boundary between the two colors.
This seems like a bug, because this same code worked fine back when I
was running PTB on a PowerBook (Motorola processor). I checked the
image by viewing it in a MatLab figure using the image() function,
and it looks fine. By the way, I am using MatLab R2008b. Please let
me know if you can help to solve the problem.
Thanks!
Aaron Schurger