I have no idea why it's not done automatically. The reason the keycolor is in hex though is because it's not RGB, it's ARGB - it includes the alpha channel.
Split a 32 bit number up into 4 parts, which from most significant to least goes alpha, red, blue, green. It's easier to see this in Hex - two characters make up each channel, from 0 to 255.
Split a 32 bit number up into 4 parts, which from most significant to least goes alpha, red, blue, green. It's easier to see this in Hex - two characters make up each channel, from 0 to 255.