Danios: Behaviour and appearance questions

So now we have four danios (never go into a pet shop with three
children and the intention to buy two fish). Two are zebra, two are
leopard. They were put into the tank yesterday afternoon.

As of this morning, two of them - one zebra, one leopard - appear
very yellow in colour. I found a photo on-line which showed zebra
danios in both black-and-white and black-and-yellow tones, so
presumably the colouring itself is normal, but is it usual for them
to change colour? They weren't noticeably yellow yesterday. The other
two fish look just the same as yesterday.

Two of them - again, one of each, but not the same pair - seem very
aggressive towards the others, chasing and seeming to nip at them. Is
this normal behaviour?

The non-yellow leopard danio looks fat - would it be noticeable if it
was pregnant? (This is one of the less agressive two, it also seems
to be moving a bit slower than the others, and less interested in
shoaling.)

PH is between 5.0 and 6.0 (bit closer to 6), ammonia is 0 and nitrite
is 0.1.

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FishNoob