What on earth does red+yellow mean? "It's about to be green"?

Yes, that's correct. It can help shorten the lag between turning green and people going as you can do your checks that the intersection is clear knowing it's about to change.

This is why modern green lights have a blueish tint to them. At least here in the US, older green lights were more of a pure green, and that meant for red-green colorblind people it was much harder to differentiate between red lights and green lights. But in this image you can see that the slight blue tint makes them pretty different.

Here I was thinking the position of the light itself was the fallback.

But then you got single blinking yellow vs single blinking red and I guess in those scenarios colorblind people just have to guess real good?

Yeah, plus approaching a light at night you need to be able to tell by color what it is signaling.

"this is why" ... Meaning... They designed it for dog drivers?!

Yes, red-green colorblind people are dogs.

(/s)