As seen on the Magnum XL-300 coaster model - here is how to dye yellow crossties red. In my opinion, this works much better than spray painting, as it coats all surfaces evenly, and leaves a smooth finish which is nearly indistinguishable from the original finish.
It does not seem to work on any flexi-rods, or any plastic with a glossy surface, but most standard rods and connectors should be fine. Just keep in mind that with dye, you can only get it to a darker color (so gray, white, and yellow pieces work best).
Materials:
1) RIT Synthetic dye
2) Yellow K'nex Crossties
3) Bowl or Pot
4) Cooking Tongs (Metal)
5) Large Tupperware Container
Process:
1) Pour 10 cups of water into pot/bowl
2) Heat up to just below boiling
3) Add 1 bottle of dye
4) Add crossties to the bowl
5) Let sit for 10 minutes
6) Transer crossties to tupperware
7) Rinse the wet crosstie with water (at least 3x) 8) Let pieces dry.