WHAT TO DO IF THE WATER IS GREEN OR YELLOW?

It's biofilm.

You will need to test for phosphates and remove them and granular chlorine, lithium or brome shock with a 60% algaecide.

Then, decontaminate.  IE: put any accessory or floating item such as a mattress, in the pool overnight, that has been in the pool in the last 3 weeks.  Wash all bathing suits, towels, flip flops, etc.  One single algae left on any item will gladly enjoy your pool and you'll have to treat it a second time.