Why do Harry Potter spells appear in different colours in different movies?

In HP 2 Expelliarmus was yellow-ish when Snape dueled Lockhart. In HP 4 it was red when Harry fought Voldemort. In HP 4 when Voldemort uses Crucio on Harry it was colourless, but in HP 5 when Harry used Crucio on Bellatrix it was red. Can they chose the colour?