The coolant used by Lexus is a rather nasty looking color brown even when new, I'm not sure why they did that. If you start the car when it is COLD and leave the radiator cap off, you will see when the thermostat opens and the flow begins. If it never does, you might consider water pump issues. But, it sounds more like a sticking thermostat to me since the Lexus cooling components are pretty dependable. That isn't much money to make it worth changing before drawing conclusions. Good luck and keep cool.