Rambam says that chupah is a yichud thats rayeh for beah. Its also assur to have beah in a room with the lights on. Does this mean if the choson and kallah go into a yichud room with the lights still on the nissuin is not chol?
First of all, I presume that your asking about a case where they go into a Yichud room where they CAN'T turn the lights off. Everybody, including the Rambam, agrees that they don't have to have a Maaseh Biah, only that it be Raui l'Biah, so as long as they can turn the lights off it would be no different than a dark room.
But even if they couldn't turn the lights off it would not seem to detract from what they are doing as having the Halachic status of Yichud. First, practically, a Mechitzah could be constructed in front (or below) the light or there is the possiblity of Ha'afalas Talis. See Shulchan Aruch 240:11 and Mishnah Berurah for all the details of these Halachos.
More fundamentally,it could be that the Isur Biah in front of a light only relates to Kedushas haZivug and therefore does not detract from the Yichud, whereas the Isur Nidah completely forbids this woman to the man and therefore does detract from the Yichud.
This is not a Psak Halachah