How would I forge a yellow light into foundry? I am sure it isn't a mod because it is in the archetect [sorry, spelling] sticky for forge.
I'm guessing you don't just mean a Custom Power Up geomerged with another object. The yellow lights are found on top of the crane and to get them anywhere else requires modding.
so #15 in this post is not modded they just geomerged a bridge at the bottom of the light to make it more noticable? http://www.forgehub.com/forum/halo-forge-discussion/45887-architecture-master-list.html#post589940
No, that bridge has been geomerged to the point where the light shows. If you see, it's looking up at it, and the background shows the crane. That's the only location you can get that effect without modding the map, as DMM White has said.