Sometimes it's just about having another colour to differentiate different tools quickly.
My hard handle ratchets tend to be red if fixed head, green if flexi, and orange for roto, it's great being able to find the one you want quickly on a busy bench!
I tend to buy other Snap on stuff in green, rather then red, for a couple of reasons. Partly because I like the colour, partly because everybody else I work with has red so I can see instantly if anything is 'borrowed', and partly (as Exceller8 alluded to) there is something special about the Snap On green - you open the box and the tools look like they're floating there!