most fish generally lose some color intensity overnight. it's just part of the resting phase. tetras are especially well-known to do this.
however, bottom-dwellers such as plecs and many predatory fish will also change their color intensity to match their surroundings.
faded colors are not necessarily indicative of stress in all fish, however fish which are typically colorful but appear "grey" (or black) are probably unhappy with their situation. i would also like to point out this is not the most reliable of standards since it relies on a precise knowledge of the ideal coloring for each fish, which is difficult to know if this ideal coloration has never been displayed.
reborn, i suggest that you take the CAE back to the store and just keep two neons in that tank until you have the money for an upgrade.
do you have a Petco or Petsmart near you? You should be able to buy a glass 10g tank for about $10, a Whisper internal filter for $15 and some sort of lid for about $10 (I have a mesh one for a reptile cage on my 10g). That's all you really need to set up a proper-sized tank for a small school of neons. Don't let them force you to buy an expensive flourescent light or anything; those are nice but you don't need one if you don't have live plants. Fish are generaly happy just swimming around in indirect light from a window or a table lamp. If you feel brave, then you can even skip on buying a lid since neons aren't particularly jumpy fish.
you may also want to get a heater, but you don't have to have one if your house is kept pretty warm (72 degrees or greater). with summer coming up, you might be able to delay purchasing a heater until things start cooling off again.
if you have a privately owned fish store around you, then ask if they can give you a good deal on a 'complete 10 gallon setup'. this should include a tank, a filter and a light fixture. most stores will also include a couple bags of gravel, a stand and sometimes a heater. this sort of package deal should cost $30-$45 dollars. if the store wants more than you can afford, see if you can get the set-up with just a lid and not a light for a little less money. you can also see about not getting the stand if one is included. however, if the store offers a 15g or 20g package that you can afford, then go on ahead and get that! the first rule of fishkeeping is
get the biggest tank that you can afford and have space for.
Whatever the store might tell you, you don't have to get an algae eater. Algae is a little ugly looking but it isn't actually harmful if you don't have any live plants; you also won't grow much algae if you don't have a light on your tank. if you do start to develop an algae problem in your tank, let us know and someone can send you some snails to eat that stuff up.
