The tap mixer is ambiguous because the labelling is unclear. A similar device, the turn signal handle in a car, is intuitive because it's positioned as a side a

The tap mixer is ambiguous because the labelling is unclear. A similar device, the turn signal handle in a car, is intuitive because it's positioned as a side appendage of the steering wheel column, and you move it the same angular direction as you would the steering wheel. It's helpful to remember that traditionally in non-legacy US suburban homes, you'd have two turn-knobs, with Hot on the left and Cold on the right. The tap mixer replaces these with one device, but the plumbing remains the same -- hot on the left and cold on the right. The label, with red on the left and blue on the right allows you to treat it like a turn signal, moving the handle towards where a traditional installation's knobs would have been, but this does require prior knowledge.