Tuner

The drone holds a steady note to tune against by ear. While it plays, the tuner only reads sound louder than the drone, so play close to the phone or use headphones.

Green means you are on the note. Yellow means you are between notes, and it tells you which way to move. If most notes say go down, pull your main tuning slide out a little. If most say go up, push it in.

Notes are named as written for B♭ trumpet. Other tuners show concert pitch, which is one whole step lower, so your C reads as B♭ there.