I'm setting FILT to a specific color that I want to use, but PHOT element changes it to red instantly. How can I prevent FILT to NOT affect from any heat source?
Leave a gap (or put an insulating element such as INSL) between the FILT and anything that conducts heat to photons. FILT and PHOT do not conduct heat directly to each other, so they are likely to be conducting indirectly, via something touching both elements.
Or set the "ctype" property of FILT, which will make it always have a specific colour regardless of temperature. ctype can be set with the console or with the PROP tool, and more details about how FILT's ctype works can be found on the wiki (Element:FILT)