I wondered this too on my 400/4 I had the same thing, that was until I refurbished the calliper and painted over it.
But I did notice if you have original rubber hoses on your bike the ends of the hoses are colour coded and have either a red sleeve or a yellow sleeve at their opposite ends and I think it's just to identify which end of the hose connects to either the caliper or the splitter/brake light switch thingamybob.
At least that's my theory.
Cheers Alan