In the same kind of dilemma I have used humbrol acrylic paints, they are hard and water proof once once dry but not as good as the original float tip colours.
I have now resorted to making me own using bits from the float store on ebay, works out about 55p per float using the foam (h160) bodies, fibreglass stem and the latest quality eyes. Because the foam is water proof no paint is needed. The green tips are an excellent colour choice for my shagged out old eyes.
I have made the mistake of buying tips and eyes at 1.7mm thinking that 1.7mm is nice and slim for sensitivity but a 1mm stem in black kind of shows through so you don't get the best flouro effect. I am about to re-buy tips and eyes at 2.5mm.
The fibreglass stem glued all the way into the tip makes the float really, really tough, although the foam bodies suffer when they hit the platform at speed after a pull out, simple to remove and replace keeping the same stem, tip and eye. Cheap floats for misers like me.