2003 prius temperature light stays on red

Hi everyone, it looks like I am back again to ask for any possible help someone out there might have. For my friends 2003 prius we replaced the thermostat and that seems to have fixed the lack of having any heat in the car. Or so we thought. The heat now seems to intermittently work when driving so maybe there's a blockage or something but who knows. We have the coolant filled up all the way in the radiator and the reserve so that's fine but now the red temperature light comes on today and was staying on as he was driving. Then it started to turn to blue and then back to red again. After replacing that thermostat my next guess is that maybe the water pump is going bad or possibly there is a sensor that is going bad that I'm not familiar with or I also wonder could there be air in the radiator system causing this? I am at a loss of where to proceed or try at this point so if anyone has any suggestions they would be greatly appreciated. Thank you for your time and help