I was asked and am wondering myself, what are materials that change color when molten? I could not find any examples online, but it really interests me. I would to get some examples which have one color when solid and a different one when liquid.
It depends lad. You see... "Colour" is not exactly a thing. It is just photons reflecting off atoms. With that in mind, there is no property of "colour". A good example to help you realize why what you're asking has no certain answer, is every metal (ie. Iron). When molten, the colour will still be of a similar shade, but because of the heat, when I has just melted, it will have a radiant red similar to fire. Another example would be water. Water is see-through. But you can see ice better than water, and Steam can be spotted with fair ease. that doesn't mean the colour has changed.
Anyhow; to my knowledge there is no such thing as a material "Changing colours" when they change from Solid to Liquid.
Also, your note on Oxygen, is correct, but, when in gas form, it is never certain what colour an element is. The way we see gas is different to the way we see solids and liquids have for example Benzol. The refractive index in Benzol is greater to the atmosphere's...
edit: Look at the comments underneath