"hot" - a high solvent to pigment (and base) ratio, and probably contains a spirit based solvent. As up-thread "Mr Color" is definitely hot (varnishes more so than paints); other hot acrylics will include Gunze Sangyo "aqueous" hobby colour, and all Tamiya acrylics. OTOH Humbrol, Revell and Games Workshop acrylics use water as the solvent. Not dead sure about Vallejo, but suspect they're water based from the droppers used. DO NOT use Tamiya "PC" range unless painting PolyCarbonate (hence PC), like RC car body shells.