I know nothing about Andorra... but images on disk can be compressed as small as you can. When loaded to graphics card memory they are converted to 24 or 32bit. That depends if you use alpha channel or not, meaning each of R,G,B,A will take 8 bits in order to be byte-aligned. So it's usually quite safe to use full color textures nowadays, unless you go for advanced optimizing things. Just know that even if the image is just 4 bit in drawing software, it may be much larger when loaded in game.