1 bit per pixel. 0 = transparent, 1 = opaque (color must be specified apart)
RGBA2222
8 bit per pixel: AABBGGRR (bit 7=A 6=A 5=B 4=B 3=G 2=G 1=R 0=R). AA = 0 fully transparent, AA = 3 fully opaque. Each color channel can have values from 0 to 3 (maxmum intensity).
RGBA8888
32 bits per pixel: RGBA (R=byte 0, G=byte1, B=byte2, A=byte3). Minimum value for each channel is 0, maximum is 255.