UV reactive 3D printing filaments are a form of colour changing filament. UV reactive filaments are modified versions of common 3D printing filaments like ABS or PLA. The polymer is modified with a UV photochromic substance, in other words a substance that can absorb UV light causing a change in its chemical structure. This structural change is what results in the colour change that we can see. The specific colour and sensitivity of a UV reactive filaments will depends on how it has been manufactured. Always check the Technical Data Sheet (TDS) for the specifications regarding UV reactivity.

It is worth noting that most photochromes have an expiration, in other words they can only change colour a set number of times before they will start to loose this ability - this is usually in the tens or hundreds of thousands and should not usually be a problem. Again check the TDS for more information on this for specific filaments.