Is it possible to add label colours to several cards at once? Perhaps some kind of select all option?
No, it is not possible to select multiple cards and apply a label to them; you have to add the labels individually.
Using the 1 - 6 keyboard shortcuts can make this easier, because you can add the label without having to open the card or the labels menu.
Selección múltiple para Trello is an extension for Google Chrome that can handle what you're looking for.
I have been doing this using the free Trello power-up Bulk Actions. Works great.
I've just cooked up myself a tool that helps in these kinds of tasks:
Batch Actions for Trello. It is still a work-in-progress, but it can already add labels to multiple selected cards.