Are they sick? Safe to put with my other fish?

My 2 neons/cardinals (I'm not exactly sure which) are lighter colored for some reason. The light has been on all day so its not that and one of them has been light colored for several days now. Are they sick? Stressed? They're in with other fish who appear fine. I'm thinking its the water quality, since neons are sensitive to water quality. I wanted to put them and the other fish in their tank in another tank with some of my other fish who just finished quarantine but I'm afraid to in case what they have is contagious.. I did a near 100% WC today, so I'm hoping it was the water quality and they'll get their color back soon...

oh, its 4 wks? I thought it was 2 wks, so I thought I could put my fish with my new ones (the ones I got 2 wks ago) today or tomorrow. the two "sick" ones along with the ones that are in their tank I've had for over 4 months (got them all at the same time), so I know it isn't a disease I brought in by adding new fish to the tank.

I hope its just stress and won't be fatal. They probably wouldn't be stressed if I could put them in the new tank, but I'm afraid to in case the are sick and give it to my new fish...

I don't know what the readings are.. I got a test kit a few weeks ago but I need my dad to help me figure out how to do it. I'll see if he will today. I've just been removing a gallon of water usually every day.

I feed them tetra food (I forget the brand) twice a day and their tank mates are 3 black neon tetras, 1 zebra danio. The water is kept at 76-78 F and Their tank is small (5 gals), which is the reason I'm in a hurry to upgrade them to the new tank. Plus I then can get more neons/zebra danios for them (since they're schooling fish and that may be whats stressing them) once I put them in the new tank. So I guess it they're stressed its either water quality, small tank, or not enough "friends" considering that they're schooling fish.