Can you not specify that the next frame's colours should be added to the current one's? Then you could do it in only 3 frames total - one for the red, green and blue channels.
I think it's just binary transparency as is widely mentioned by commenters around the web. I found this in the spec:
"viii) Transparency Index - The Transparency Index is such that when encountered, the corresponding pixel of the display device is not modified and processing goes on to the next pixel. The index is present if and only if the Transparency Flag is set to 1."
"viii) Transparency Index - The Transparency Index is such that when encountered, the corresponding pixel of the display device is not modified and processing goes on to the next pixel. The index is present if and only if the Transparency Flag is set to 1."
http://www.w3.org/Graphics/GIF/spec-gif89a.txt