[Florablunder]

I like to fill our front window boxes with bright red geraniums* (makes me think of holidays in France).
But I've found that they don't seem to stay red long term - over time they either veer off more towards orange, or gradually mutate into fuchsia/magenta pink. Has anyone else found this? Because it's quite sheltered by the windows, the plants seem to survive all year round - so, I'm loathe to have to buy a whole bunch of new plants just to get the colour right. But, maybe that's what I do need to do? Any ideas??
(*and I mean of course pelargoniums).