The males tend to show up shallow first, usually around 63-65 degs. Females make runs up shallow, then back out deeper in stages. around 68 degs they have usually began spawning but just like everything else, weather, cold fronts, water clarity and length of day all have to come together at the right time. Last summer we caught spawning crappie in one res in NW KS for literally 2 months, but never big numbers on a given day.
I will say every lake is different but here, we see crappie spawning in 2-5ft of water with generally 3ft vis. You have to have the right structure in the right depth in any lake. Water conditions can change that depth considerably from one year to another.
Most years we find them holding-prespawn-in 15-18', when it's game on they will be likely in 4' if conditions, habitat, etc are right. Males of both Black and white turn black before spawn and can stay that way for several months, females have no color change.
