Resize with a gesture
Swipe up or down on the circle to make it larger or smaller. Adjust it so the area contains as little as possible of any neighboring color.
Tap to measure
Tap the circle to identify its color, or use the Measure button. Hold your iPhone still for a moment while the camera adjusts.
Press and hold to calibrate
A longer press starts calibration. First aim at a matte, neutral gray or white surface in the same lighting. A separate Calibrate button is also available.