I guess the screen shot
from familay Tree - how to create his own bridge applies here as well.
Unfortunately, it is not possible to specify gradient colors in yEd's GEDCOM Import. You will need to adjust colors in a post-processing step: Either select nodes by type (male, female, family) using the "Current Elements" palette section and then change colors in yEd's properties view or use yEd's properties mapper to adjust colors as required.