It’s usually better to spray several thin coats of color and let each layer dry. Build up to the shade that you want. If you try to color a very deep color all at once, it’s gonna take a very long time to dry. Also check that your ambient humidity is not too high, the air needs to be dry or else the color won’t dry, an air conditioned room is the best solution for this.
Also, as 3DSweets correctly pointed out, if you’re trying to paint white fondant red (or any other deep shade of color), that’s A LOT of coloring… You should consider kneading a gel color into the fondant instead or buy ready colored fondant. If you still need the airbrushed look instead of a solid color, you can still go over once or twice with the air brush to get that look after, but only in thin layers.