Given these two limitations, Starship currently carries very little fuel. Think of it like a mostly empty soda can with a few sips sloshing about the bottom. Adding 3 Raptor engines to such a tank makes them way too powerful to run at 100% thrust the whole time.
On the launchpad, all 3 Raptors start off at full thrust until well clear of the pad. At that point, one engine at a time starts throttling down gradually, so as not to go supersonic or exceed the 10 km flight plan. A throttled engine does not burn as cleanly or efficiently, which is what results in the orange exhaust plume. Same as your campfire, blue flame is hotter than orange/red flame, and the bluer exhaust engine is therefore burning hotter and providing more thrust.
As Starship gets closer to the 10 km peak, it shuts off one engine at a time, and finally throttles the final Raptor down as low as it can go. Note that Raptor engines have a minimum throttle that is ~20% of max thrust, below which the only option is to shut down.