The color components can be set to any value from 0 (no intensity) to maximum intensity (255).1
It is important to note that even fully transparent pixels may not have their red, green, blue components set to 0. This is useful if you want to add a value to the alpha channel using one of the color transformation matrices. In that case, using all 0's would generate a black color.
f_red = orginal red * f_alpha / 255
This format was added to be mostly compatible with the old PNG format (XRGB).