Where can I find tin?

[Johannicus Mechtillery ™]

What color on the ground should I look for to find this mineral?

[Tux]

looks like silver but more to the question, why?

I dont think it has a purpose different from copper honestly. I would go straight to iron or steel use copper and tin for things that are not important like metal parts

[Johannicus Mechtillery ™]

How does silver look like on the surface?

[sonicthe]

Tin will look very shiny, either as a chunk to pick up or as a deposit.

[Wastefield]

Yeah, dark grey or even black-like. When it reflects light from the Sun or a torch, it makes a distinct "black glow" you can't miss.
And actually bronze is one of the first things anyone should try to get since copper & tin are very common everywhere and it makes the 2nd best tools in the game! A recent update buffed bronze tools (and weapons) to have the same highest efficiency as steel, so they're weirdly like the best tools in the game, only with less durability! Which doesn't really make a difference since the metal is 100% recyclable after the tool breaks - as if there still wouldn't be plenty of bronze anyway...
It was changed probably because people skipped bronze to jump straight to iron. But now this just made iron absolutely useless instead. It's worse than bronze if you didn't notice! No one should ever craft an iron tool or even use one that they've looted: bronze and steel chops a tree & builds a floor in 1 hit, iron needs 2 hits and effectively makes you twice as slow...