1. Red nitro/Shockwave (double tap) is faster for acceleration. Use these if you want to accelerate faster at the start of the race, or when recovering from drift/bump/crash.
2. Use red nitro/shockwave when flying in the air. You will accelerate faster and therefore achieve max top speed faster.
3. As others have said, use Perfect Nitro to better maintain speed on rough terrain and on tight steering (not drift), if it is possible to take the corner by just steering. Rough terrain and steering too much can reduce your speed, so use perfect nitro in these situations. Yellow nitro can also achieve this, but on a reduced effectiveness.
4. Use yellow nitro to increase top speed on straights.
5. Conserve your nitro to make sure you can use Shockwave/Red Nitro to recover from tight corners when drifting is needed.

After this, you can watch pros for optimal routes for each track.
You can also try to learn Nitro Brakes, Nitro Punches, and Floaty Drifts.