Shine blue light upon them.
Work Step by Step
Blue is the complementary color to yellow. Blue + yellow = white. Under normal illumination, yellow costumes absorb the complementary color to yellow (which is blue), reflecting yellow light. The yellow costumes appear black when illuminated by blue light, because the cloth absorbs that light, and there is no "yellow" light to reflect. This is discussed on pages 505-506 and 509-511.