It's 32768 colors
Let me point you to a nice spec on the GBA:
GBA Tek Doc
also try this to be doubly certain:
Mappy Tek Doc
Anyhow.. back to what I was blabering about (grin).
Because it's color resolution is less than the 24 bits typical of FMV's etc (IE it's color rendition is lesser) I don't see compressing the data to be too much of an issue. 320x224 x24 bit @15fps to 240x160x15 bit @8fps and 160x128 is more likely the resolution so it's more than adequate.
The need for space though for all this, would be the biggest challenge. Still it's doable by splitting the ROM space into 4 sections and using part of SRAM space for switching ROM segments.
Example.. base section fixed at 64M (8M program space) and the upper 3 segements are memory maped.
each map is say 8 bits? that makes 256 * 8 or 2Gigabyte of memory mapable data. In other words, not a big deal. As for cost of doing something like that, ehhh.. not cheap. Need extra hardware and of course more chips for ROM space. Say each ROM costs $5 each.. and thus the user $15 for each ROM and the cost of the title(costs are pretty high from manufacture to consumer). Let's say 8 256Mbit chips used (256Mbytes of memory) that's $120 for just ROM alone plus maybe $40 for the normal overhead. Comes out to $160 or more (yikes). Rom media is expensive.
Cyb