Colour Codes
Same reason that electrical wire are coloured. Instead of labels you have colours.
You can also make the fluids coloured with dyes so if there is a leak you can tell which system is leaking.
Also because all systems are not the same in other words to stop service people connecting the wrong tubes together.. Yup really is that simple.
If I am not mistaken I think A388 only has two systems, not sure though.