A tm is just a disk storing elemental energy. When it touches a compatible pokemon it will release this energy and break the disk. Later Tm's were developed that don't break and are quickly rechargeable.

>why colored
The color represents the elemental energy stored inside

>why disk
Metals are good conductors which can be charged quickly. If kept in a insulative Tm Case they can keep their power indefinately. Additionally disks are thin, and easy to distribute compared to gen 1s collars.

>what are Hm's?
Hm's use the same process as modern Tm's of not breaking from use. This was very expensive technology, but has since become common place on all Tm's leading to the Hm's eventual disapearance.

>what are TR's?
Tr's are a nostalgic new product marketed as an alternative to Tm's. They have more obscure or expensive/hard to produce moves.

Please stop with the tm meme. IRL we can charge disks with energy already. This shit is canon as seen in books, anime, and FRLG