These are "Three public mail boxes in Santa Cruz on the island of Tenerife." Porto, in Portugal, has
red and blue mail boxes (
and they even talk to one another). Whilst I am now used to our
yellow mail boxes, I had not been aware that we have them in other colours too, but
Jess Artem also just pointed out that "Sometimes they put a red one next to the yellow". Intriguing. My feeling is that they might be for different destinations (inside / outside the district, or Spain), rather than for slow or fast mail, but I could be wrong.
Usually it must be Yellow: standard mail, Red: urgent mail, Green: only for postmen use (they use them to pick or leave bags)