"32-bit colour" is for all prac perposes is just 24-bit color that is aligned so that the color information for each pixel in the bitmap starts every four bytes vs. every three bytes.
The only reason this is done is that on the Intel 32-bit PCI/AGP/Memory data bus its more efficient to get at data that is "32-bit aligned".
(with 24-bit data you often actually have to read the pixel\'s data twice, and then mask for the bits you are interested in)
The fourth byte is normally wasted.