The light will be white as Poser doesn't have caustics, which would cause colour change to the light. If you used a green light, you could fake colour tranference.

This thread may help with the gel effect, http://market.renderosity.com/mod/forumpro/showthread.php?message_id=928454&ebot_calc_page#message_928454  it's by SnowSultan, and it utilises his Globall lightset in the freestuff here. You can always change the trans-map to something else.

Here is a quick pic showing it in action, rendered in Pro-Pack. The spotlight is casting an orange-red light, hence the orange-red everywhere, also, the gel-square would ideally be out of camera view for proper effect, it is in view to show it operation.