Woah! Are the Orcs coming back to MTG this year? Its been a long time. I'm really confused why they removed this creatures in the story for a lonf time. Anyway, I love the art, I'm a goblin and orc art fan. I love those rainbow thingy,and that gives me an idea to practice my light effects.
Painted a ray one third the width I wanted in a yellow tone and set the layer to Hard Light if I recall. Then duplicate that layer, move it over a little and shift the color to red. Duplicate the yellow again, move to the other side and shift to blue. There very well may be a better way to create the effect but that's how I did it.