You'll first have to decide on how many bits each channel gets in 32 bits (last I checked 3 does not divide into 32 evenly. You could also tack an alpha channel on the end if you're using 24 bits to begin with). Second you'll then have to scale each channel into however many bits you allocated for it. I don't think ANDing each channel with 0xF0 will do that, though.
JoeG