You could always spin the vert pos knob and see which trace moves...
Which is exactly what I do, unless I just poke the big glowing buttons to turn one off quickly and tell them apart that way. Doesn't take away from the fact that either nobody gave any thought to making life easier for those of us with vision deficiencies, or they did and then decided not to bother. Nor, come to think of it, does it help in screenshots...
The only particularly good reason I can think of to actively not do it, presuming it was thought about early enough in the design process that the ASIC supported it, would be the colour coding on the front panel; I can see someone deciding that having the traces not match the channel selectors would cause confusion/be ugly/something else an affected user wouldn't care about. Now, I think some of the (reassuringly expensive!) R&S scopes use RGB LEDs to make their front-panel buttons match the user-configurable trace colours, but I could be mistaken.
(When all's said and done, though, it's no worse than using an analogue 'scope or a monochrome DSO, as the OP observed. At least yellow and green are both visible, even to the majority of sufferers. ISTR some Chinese scopes use yellow - or maybe it was green? - and red, which would render one channel totally unusable for some of us; red is so dark as to be almost invisible against the black background.)