There are several ways you could go about pruning the trees and which you chose depends on your situation.  Do you want the plants to be an ornamental focus of your landscape? Or are you hoping they will act as screen?  It’s hard to tell from the picture but from your description of different colored leaves and fruit I suspect the understock which the crab apple is grafted onto is suckering.  These suckers- growth coming up from the soil- not from the main trunk could certainly be pruned out.  The broader pruning of the trees is a more complicated subject and I would be happy to talk it over with you if you want more in depth information.  There are some general rules to follow:  1) Always prune back smaller branches to larger ones not the other way around.  2) Don’t prune too much in any given year- depending on the health of the plant only remove 10-25% of its foliage per year.3) The best time of year to prune in is the spring just before the plant leafs out4) A bacterial disease called fire blight can infect crab apples. To minimize the risk of this disinfect your pruners in a solution of bleach between cuts.Here is some more detailed information on pruning young trees (this will not all apply to your trees because they are not shade trees but it has a lot of good information:http://www.cmg.colostate.edu/gardennotes/614.htmlAgain feel free to give me a call if you would like more information.