Sprite Diffuse shader extrange behaivor with more than 2 light in WebGl.

Hi, I am working with the sprite diffuse shader and I make a scene where 3 light can illuminate the object. The light are red, green and blue. When the three lights are near of the object with the sprite shader, the object is paint with a purple color. But when one or only two lights are are near it, the object is paint right.

Any idea? Thanks. Esteban.