One species wearing many colours
Almost all the coloured dwarf shrimp offered for a single tank — cherry reds, sunny yellows, blues, oranges, blacks — are one species, Neocaridina davidi, line-bred over generations from wild shrimp that are translucent brown. Colour lines are not separate kinds of shrimp in the way different fish species are; they are families selected for one pigment trait, and they recognise each other perfectly well as mates. That single fact drives everything else in this article: any two colours sharing a tank will cross, starting with the very first female to carry eggs, and no arrangement of plants or hiding places will prevent it in a tank where they all graze the same surfaces.
What the babies actually look like
Crossed offspring do not average into attractive blends. The colour traits depend on several genes at once, so mixed-line young typically hatch pale, patchy or plainly wild-type, and each unselected generation slides further toward the ancestral camouflage — a colony that began as jewel reds and blues becomes, within a few generations, a tank of mostly clear-brown shrimp with the occasional coloured throwback. The shrimp themselves are perfectly healthy; only the colour project has failed. The speed surprises people, too: each female carries dozens of young and the young breed young themselves, so an unchecked crossed colony does not drift slowly — it can tip from mostly red to mostly plain within a few generations. This is also why a coloured colony should never be topped up with a mixed assortment bought for variety: one infusion of foreign genetics colours nothing and dilutes everything the line had built up.
Keeping a line true
The working rules are simple. Keep one colour per tank — or per divided, shrimp-proofed section — with no exceptions for a pretty individual of another colour, because one exception breeds. Remove off-colour young early, before they breed in their turn: in an actively breeding colony, this culling is the unglamorous maintenance that keeps a deep red line deep red, since the palest shrimp in the tank reproduce at exactly the same speed as the finest. Buy additions from the same line, ideally from stock bred true over generations, and settle new arrivals in a separate container briefly so you can spot a wrong-coloured stowaway before it joins the colony. And know which shrimp genuinely cannot cross: Caridina types such as crystal shrimp are a different branch of the family and will not interbreed with Neocaridina — though their water preferences differ enough that sharing a tank is a separate question altogether.
Records are the breeding programme
A colour line is a record-keeping exercise as much as a shrimp-keeping one: which line, from which source, when off-colours began appearing, which young were removed and when fresh stock was added. Without notes, a keeper cannot tell whether a fading colony needs new same-line blood or simply stricter culling, and guesses wrong in both directions. Logging the colony's line, sources and culls in its KinsLe Pet Passport turns guesswork into a breeding record — and if the colony ever moves to another keeper, the line's history travels with the shrimp.