Fish poo is usually the colour of the food they've been fed. If the shop was using orangey red food, the poo while that food was passing out of their system would be red. If you feed vegetables like peas, their poo will be green. The poo that might be cause for concern is white stringy poo, though I've never been absolutely sure what stringy poo means. That can be caused by internal infections or parasites but is often accompanied by the fish looking skinny. While they are new I would wait till they've settled in before worrying.
And platies do poo a lot!
The behaviour you have seen is common in newly purchased fish. They have been though a lot, from being in the shop tank and gawked at and had a fish net put in the tank every so often; being caught in a net themselves; then put into a new, unfamiliar tank with different water than they are used to. Some fish get over the stress quickly, others take longer. And it is common for fish to not realise that your food is food if the shop fed something different.
But, your nitrite reading does give cause for concern. What are you using to test with, strips or a liquid tester? And do you have an ammonia reading? I remember you said you were doing a fishless cycle - did that finish OK with 3ppm ammonia dropping to zero ammonia and zero nitrite in 24 hours?
Strips do tend to be less accurate than liquid testers, and an ammonia reading would help.
Because I don't know what your ammonia reading is I will recommend doing a water change. The nitrite level needs to be below 0.25, so with a reading of 1.0, you need to change at least three quarters of the water in the tank. With that amount of water, you need to get the temperature of the new water the same as the tank or the temp difference can shock the fish - comparing the temps with your hand is close enough. And don't forget to dechlorinate the new water.
Incidentally, you really could do with another female. Males harass females constantly and it is recommended to have at least two females for every male to spread out the harassment. But not yet, not until your readings for ammonia and nitrite stay at zero.