How can I know which colour is Front speaker and Rear Speaker?

If you assume the colors don't change en route (say, when different wire harnesses join), you could pop the trim and look at the speaker connections, though that could be a lot of work. (If the colors do change, you'd end up digging out the various wire harness junctions too, more work.)

If you just want the right answers without the work, the wiring diagram is the one sure-fire place you know you'll find them, techinfo.toyota.com.

-Chap