First we're going to build a simple timer with visual display for pacing the breath, then elaborate the display and perhaps add other sensory modalities such as auditory and haptic signals.
I'm using four LED's in a vertical column. The bottom LED represents a complete exhale and the top LED represents a complete inhale. To provide pacing, I fade from one LED to another. The two middle LED's will indicate that we are between the two extremes.
The next step is to decide how to factor color into the display.