Green charges on blue (I agree, very poor contrast) may have originally been intended as charges "proper" on a blue field , which some later heraldista chose to blazon as "vert" rather than "proper"—e.g. if the charge is a lizard or tree or parrot or whatever.  If thought of as "proper" then a good artist would look for the best contrast he could concoct on his palette—light green or pastel or whatever might show reasonably well.  Once translated to "vert" the range of artistic license is llimited somewhat—and entuirely lost if the painter has no heraldic expertise.