What color grapes are "Malaga grapes"?
I also googled it and it seems that sweet white or green grapes are, indeed, probably the closest answer. Since it is a fruit salad, you may want to choose whichever of the grapes is sweetest rather than choosing by color.

Winemonger's page on malaga says that it is a white wine grape and can be called by the following names:


Hunter Riesling (Australia) 
Blanc Doux / Chevrier / Semillon (France)
Boal / Wyndruif (South Africa) 
Green Grape (United Kingdom) 
Semillon (United States)


The wikipedia link for Sémillon describes that as a sweet, golden grape.

This interesting Chicago Tribune article from 1890 about what people in NY fruit markets were calling Malaga grapes also indicates that it is a "white or pinkish white" grape, and not the true Malaga grapes, which that article claims were too delicate to ship.