This is a bug that was present through the entire run of Poser 12. It went unnoticed until Poser 12 development ended. It has been fixed for Poser 13 but that doesn't help you. There is a work around but it's a bit cumbersome.

The failure was in the way lights store the animation data for the color and intensity of lights. First is to delete the light, recreate it and set the color before you render the scene with that light present.

Next if you have already rendered the scene you can usually "jiggle the right lose wire" by disabling the Animation on the light's shader node in the material room. Select the light and click the little key icon next to the color and intensity icons and remove the check from Animated..

This will cause poser to re-generate the shader nodes for the colors of the light. It won't stay fixed because the animation will pop back on as soon as you render the scene. (Because the way lights colors animate is totally borked in P12.)

So if you change a light's color and it doesn't respond in a render first try toggling off the animated key in mat room. If that doesn't work try deleting an recreating the light.

This only really affects color lights. if the light is a white it doesn't proc the bug. That's probably how it went unnoticed for so long.

Hope this helps.