If you're going for a very temporarily colored mohawk, try aquarelle crayons. The kind you can also use as face paints. You'll get a stiffening effect as well as color.

Both Manic Panic and Special Effects are considered semi permanent. I dyed my hair green with both Manic Panic and Special Effects. The color fades on me but never really washes out. When I needed Soccer Mom hair, I colored over the green hair with a deep chocolate brown, but in some lights, green highlights still show through.

Another option for very temporary is food color.