TL;DR If you’re seeing color banding in your exported video, it’s almost certainly because you’re using an encoding profile that specifies a color depth of 8 bits. Use a codec, such as H.265 / HVEC, DNxHR or ProRes, that supports 10 bit color and configure it to use a 10 bit encoding profile.
I’d seen volume gradients used in a few places and not really understood them. Houdini help says: “the gradient is a vector pointing in the direction of increasing volume values.” Hmm. Nothing if not laconic.
The discourse around digital color is complex and confusing. If you care about color at all and you work in digital, you should probably spend at least a whole day of your life on The Hitchhiker’s Guide to Digital Colour, which gives several messianic fucks about making this difficult stuff clearer.
Converting a mesh into a highly viscous FLIP fluid in Houdini, then selectively lowering the viscosity of certain areas by raising their temperature, is a fairly simple way to simulate a melting object, with reasonable art-directability by means of noising and animating an invisible heat source.