First you have to consider what causes the spiral arms. Despite their visual appearance, they do not contain significantly more material than the "gaps" between them. They just contain a higher percentage of bright hot stars. They are caused by density waves passing through the galaxy, which induce star formation. The larger the star formed, the brighter it will be, but also the shorter its lifespan. Spiral arms are where you are going to find young bright stars. But smaller dimmer stars are also formed, and in greater numbers than the larger ones. They also live longer. By the time a density wave leaves a given part of the galaxy, all the large bright stars have expended themselves, but there are still lots of smaller dimmer stars left. These are the stars that now predominate the regions between the spiral arms. The "gaps" look darker for the same reason that sun spots look black compared to the surrounding Sun surface. Sunspots can be 4500 C in temp which, if viewed by itself, would be on the Cool White region of the color temperature scale. A collision with Andromeda will likely produce some density waves of it own, creating new regions of star formation for a time. As to what the final result may be, it could end up as an elliptical galaxy, which will be featureless (no disk shape or spiral arms).