What can be substituted for green onions when making crab cakes?
In general, shallots, leeks, or chives would be milder than regular onions and probably a closer approximation to green onions, though all are usually harder to find than green onions.  Leeks and chives have the advantage that they also have a similar color to green onions.  The sharpness of all of these substitutes (onions included) can be tamed by cooking, but this would affect both texture and color as well.  In crab cakes, the color probably matters less than the flavor, but you could mix cooked shallots or onions with raw leeks or chives to get both color and flavor.