I was told by a friend that whenever neon tetras are resting their colors fade away. This morning I took a look at my tetras before they started moving around and the colors did in fact seem to have faded away. Is there a reason for this happening?
Fish have cells called chromatophores that produce the pigments that give coloration or reflect light. The colour of a fish is determined in part by which pigments are in the cells (there are several colours), how many pigment molecules there are, and whether the pigment is clustered inside the cell or is distributed throughout the cytoplasm.
Another thing is, when fish lose their colour, the chromatophores space out pigment granules farther apart, and their colours appear more dull. Drawn closer together, the colours appear more bright.
So basically, your fish when resting is conserving its energy, there is no need to maintain their colour.
BUT BEWARE, there are many reasons for the fish's colour to fade out in other cases such as stress, bad water quality, when introduced to new fish or when diseased. So if this only happens when there is absence of light or when resting, it is perfectly normal.