For semen, try to do white on a low opacity as your base and then some strong light highlights and edges. Also cum has different consistencies, it pools in some places, where it becomes thicker and opaque and in other places it’s almost completely translucent. Try to keep that in mind! Doxy did a great tutorial on cum: http://prismblush.com/cum-paint-with-me/

As for precum, I’d just do it like normal fluid. Slightly darker color than what’s behind it and some white highlights.

I hope that helped!