If you can get any colour other than green (and
black, I guess) out of an
unmodified IBM 5151 monitor, I will eat a classic computer!
If you connect it to a high voltage source, maybe you will get a brief
flash of yellow orange, red (and maybe blue) out of the back... followed
by some puffs of grey out the top.
Maybe... If you apply the HV to the mains connector, I suspect the only
thing that would fail would be the fuse (the 5151 has a linear PSU with a
nice fat mains transformer at the input).
If you apply it to the signal cable, well, it'll probably take out the
input buffer chip (a '04 IIRC, I can't be bothered to go and get the
schematics). Maybe some smoke. I doubt you'd get many interesting effects
though.
In any case, the monitor is not unchanged at the end, so I would class
that as a modification :-)
-tony