Spanish Guinea is tiny, and the rest of the territory at the time was right next to Spain: northwestern Africa (Morocco and Spanish Sahara, etc). The civil war actually basically STARTED in Morocco by the Spanish Army of Africa.

So the answer to your question is kinda fascinating: one of the questions that sparked the civil war WAS the question of letting the colonies go. Francisco Franco was stationed in Morocco when he joined the rebels.... so actually to answer your question: the colonies ended up taking over Spain (is one way to look at it) with Franco's African Muslim troops being the most effective on the Nationalistic side. Basically the troops in the colonies supported the rebels. Combine that with the fact that the colonies were close, and not on the other end of the world, and it's a totally different situation than if Spain still controlled parts of the Americas or Philippines etc.

No idea how much of a risk they were at losing Spanish Guinea.. but it is just a tiny speck of land and they had just finished 'pacifying' (murdering hundreds or thousands of the Fang people, for instance) in 1929. I can't say without speculating what the actual chances of the locals breaking free were (my speculation is not good only a couple years after being 'pacified').

..and while I'm just listing historical facts, it's still kinda a superficial answer. I don't know enough about the governments of the colonies to understand the risks, but obviously, if the rebels started in the colonies, it was more securely held by the rebels than the Mainland even.