Destructoid reviews Sonic Colours, my review, our conflicting opinions...

[Conegamer]

tolu619 said:

All I want to know is: after you obtain a wisp, can you use it at anytime? Can you switch wisps mid-level? Any other thing I should know about how the wisps are used?

You can have absolutely as many wisps as you like, swaping at any time. Heck, they're optional anyway!