You can buy colour stuff separately (and that's how labs do it, 50L+ at a time) but there are so many components that the manufacturers tend to sell kits. It's the easy way to make sure you aren't missing a component.
A proper C41 process requires a developer (3 concentrates), bleach, fixer and stabiliser. That's 6 different concentrates you need to mix up into 4 baths, which means that buying separately is annoying, especially if you don't know exactly what you need. E6 is slightly more complicated again.
But…once you get your processes sorted it is no harder than b&w.
Just a bit more labor intensive.