Black wash does not work with yellow. You actually want a chestnut or flesh wash to get the best effect.
Seconded. I have been getting good results using GW's Dark Flesh to shade yellow.
To make up for the lack of panel lines you could try to apply a few controlled washes of Dark Flesh where natural shadows would be, for example towards the underside of the torso or below the turbine/tank/whatever thingies, the sides of the weapon pods on the arms, ... This would break up the yellow and add more depth to the miniature.
Impressive work with a difficult color nonetheless :) I really like how the infantry miniatures give a sense of scale. They really make the Marauder look huge.
