Thusly FishNoob Spake Unto All:

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.

This:
http://mikes-machine.mine.nu/specime...o_DSC_0324.jpg
is what a zebra danio looks like.
Leopard danio is believed (it is not certain) to be a breed of zebra
danio, and has much the same base coloration.
Neither are ever, AFAIK, white.

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?

Yes. Males are somewhat territorial.

The non-yellow leopard danio looks fat - would it be noticeable if it
was pregnant?

Yes, females are much plumper than males.

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

That's a low pH. Unless you are breeding sof****er fish like neons,
you'd have greater margin of safety if you put a couple of limestones
in the tank (which'll buffer the water and give a pH of 7.5). Now, 5
isn't lethally low, but many fish will have problems reproducing at
that pH, and at 4.5 some fish will start to die. You also can not keep
snails or shrimp at a pH under 6.