If you have no damper, you probably have a ventless gas logs system, which is dangerous and is outlawed in sleeping rooms according to building codes in many communities, because of the risk of oxygen depletion and/or carbon monoxide build up. Consumer Reports has a good article on the subject available free of charge on their website.

For safety, you should keep a window open a crack, which you may not want to do because it will bring in cold air, but which will compensate for the harm that your gas logs can do to your indoor air quality.

If the rainbow on the glass is due to excessive heat, the only solution is to shut off the system periodically to prevent overheating. Direct vent systems, which are safe and energy efficient (an a good deal more expensive than the unvented gas logs you have) usually come with a thermostat that turns the system on and off periodically so overheating does not occur. Without a thermostat, the only solution is to turn it off and on periodically yourself.

maryo says the fireplace is vented and there's no reason to suppose otherwise. The only gas fireplace that have dampers in them are mostly open fireplaces that are rather old --- dating back to the 1960s-to 1970s in most cases.

Newer fireplaces use other design features to control the draft through the fireplace.

As to what might be causing the rainbow effect--- I don't know. I never saw that happen in a gas fireplace.

I'd call the retailer where the fireplace was purchased and ask them if you know which outfit it was. Other wise I'd call the fireplace manufacturer and ask for technical services, which usually connects you up to the engineers or service reps who are experts on the equipment.

Because the glass is tempered. End of story. Nothing is causing is besides it being made that way.