Finding and installing a carbon monoxide/natural gas detector

In our area, local codes made at least one CO detectors mandatory in all condos and rental units. I have had to replace dozens of standard smoke detectors with combo units in hard wired/battery back-up systems. Most every manufacturer makes a CO/Smoke unit that will plug in directly to the existing system. A hard wired replacement is far better than a stand alone battery operated unit, as it will alarm all units in the system if there is a problem.

Having the system monitored or not monitored makes absolutely no difference on how the system works inside the house. All monitored systems will work fine in the house even if the monitoring system is off.

It is (very) surprising that the gas company (Xcel) says "everything's fine." This document from a different gas company certainly calls out non-blue flames as an issue that needs to be resolved. As does this site. And Xcel themselves.

Happening on all appliances does seem a bit less likely to be "and suddenly the air adjustments on all three went whacky" than a contamination issue - possibly water in the gas line?

I wonder how your furnace tech visit went, and what your neighbor's flames look like.

Editing post-comments: Ultrasonic humidifiers make small water droplets (without evaporating the water first) and then those small droplets evaporate while floating around - if the water contains salts or minerals, any salts or minerals in the water will end up floating around the house as tiny specks when the water evaporates from the tiny droplets. Those, evidently, are what is contaminating your gas flames as they are sucked in along with the air required to burn the gas. Old-style "sponge" dehumdifiers (should) only result in water vapor in the air, as they actually evaporate the water (and this eventually leads to a build-up of scale and salts in the humidifier and its sponge element, as those do not evaporate.)