Multi-color is probably not really possible at a reasonable price point because you'd need twice as many vias and traces in the same space for RGB compared to just R. You _can_ get it manufactured, just not in low quantities at a price anyone would be willing to pay. There are 1.1x1.1mm addressable LEDs available[0] which should be quite doable, but those require quite a high voltage and have an unacceptably high idle power consumption: a 52-LED matrix would draw 15mA with all the LEDs off!
As to single-color non-red: the main advantage of red LEDs is that they can operate on a very low voltage. The exact same board with blue LEDs would have a significantly shorter battery life - if it's even possible at all. These earrings are probably using two SR521 batteries, which start at about 1.55V and discharge to about 1V[1] - so the earring is operating on 3.1V to 2V. You can get red LEDs which work with as little as 1.7V, but blue LEDs need about 3V to operate _at all_. You'd either have about 1/3rd of the battery life, or you'd have to add a third battery.
Great question! Each individual photon carries more energy, so I'd assume they do indeed consume more overall power. But maybe eye sensitivity compensates for that?
true, but it's challenging to get a boost converter, even a very simple one like a joule thief, into the space of this earring; i think it's too small for a millihenry. using a larger number of smaller batteries might be a better option
It looks like you can get reasonable SMD boost chips and pack them and their entire support infra in <10mm https://hhtronik.com/product/uboost/ which is about the same as two batteries as used in this project.
