I've never grown before, but from what I saw on youtube, leaves can turn that color from a build up of salt in the soil. They solved it by filling the pot up with water to the brim and letting it drain. Do that about once a week.
I have the same problem. Two days ago I switched from veg to flower. The leaves tips started to yellow overnight. I'm using hydroponics and the UFO led light, so I know heat is not a problem. For nutrients I'm using gen hydro maxi grow and maxi bloom. I attached a few pictures. Any advice would be appreciated. thx!