Will re-seeding different areas of lawn result in different colored grass?

Would I end up with areas of the garden that have different colored grass if some is the existing lawn and some is newer grown from seed?

 

Yes, the new areas would probably look a bit greener and "shinier" but they'll all merge in together soon enough, certainly by next season. The weather would affect the speed of merging in - you should be doing your best raindance! You can stand there all day with a hosepipe but nothing beats prolonged, steady rain for making grass grow fast.