Hi. Are you sure the "Titanium Tank Chromatic Cardioid 2020" paint variants are correct? Extract the .vmt files from TF2's .gcf again, and check if the "$color2" is different than "$colortint_base". If it is, you must copy the value from "$colortint_base" and paste on "$color2". Remember, TF2 only reads "$colortint_base" in-game, never "$color2".
If the .vmt does not have a "$colortint_base" but has a "$color2", you must remove the "$color2" line entirely. TF2 doesn't read "$color2", but a material doesn't necessarily need to use "$colortint_base". Most of the time, items without "$colortint_base" are part of cosmetics that are not paintable.
I believe that the "Titanium Tank Chromatic Cardioid 2020" metal and reflection materials are "vmtname_1.vmt" and the paintable part (the heart) is "vmtname.vmt".