Cherries are a color morph of a brown colored shrimp that are missing some darker pigments. Breeders have selected them over many generations. The redness of your shrimp will depend on the genetic line your shrimp are from. Because red cherry shrimp (RCS) lack brown pigment genes you will not tend to get brown offspring unless they cross bred with other color morphs of the same (or similar) species. however over time you may get offspring that are speckled, pink or even completely transparent rather than red or mostly red. It is wise to remove females that have reached a reproductive age and still have little or no color, and let your reddest females contribute more to the next generation. It is also wise to occasionally add introduce new shrimp from good genetic lines to keep the color of your shrimp bright and avoid inbreeding.
Pigmentation exists mainly to camouflage the shrimp against its background and assist in predator avoidance. Darker backgrounds such as shadowed areas, black gravel or sand, dark wood and dark plants will encourage stronger color. The presence of curious fish can also provoke previous pale shrimp to color up, although aggressive and predatory fish should obviously not be kept with drwarf shrimp. The shrimp hiding in the picture below (can you see her?) was totally colorless the previous day, but was then moved into a tank with a female betta fish.
The shrimp makes its pigments from components in its food. It pays to allow shrimp access to natural algae and plant matter. I would also recommend a diet that includes algae (algae wafers or spirulina powder), invertebrate diet (such as Hikari crab cuisine) and a color-enhancing fish food (such as Tetracolor).
In most cases fully red shrimp are old shrimp. Most shrimp will color up significantly if given time to fully mature.
Keep these thee areas in mind and you should have your red cherries living up to their name in no time!
Why aren't my Red Cherry Shrimp red?