Guppies – Why? Magic?

I recentley bought 1 male guppy and 3 females.
Soon after we transfered them into are cycles, spacious 15 gallon tank, the male started to chase the females. They have this tank all to themselves…
One of my females has the darkest gravid spot ever. Small but black, really black.
Is she pregnant? Does she have to be fat to be pregnant?
She is about 2-3 cam in length.
The male has also stopped chasing them, why?
He is very healty and they all stay together. Could he be protecting them…
Also: My guppy always changes colour. In the morning his plae blue, when I come back from school, he is sparkling green and yellow. He looks like a completely different fish, why?
It happens everyday. The females are ok, its just him.
I turn the lights off before I go to school, come back ant 3:30 and then turn it on, he has gone green.
Is this normal, have I got a rare guppy.
My tank consists of, pump, filter, heater, fine gravel, 2 real plants, 2 fake plants, ornaments.