Why is there a color change when ethene reacts with bromine?
Because neither ethylene nor ethyl bromide have a characteristic colour.
On the other hand, bromine has a characterstic red-orange colour. When bromine reacts with an olefin (which are typically colourless), the orange colour dissipates as it reacts with the bromine.
Typically, we use orange-coloured bromine water, and the reaction gives an halohydrin:
Bromine is the electrophile, i.e.
Bromine is the one of the most corrosive substances you can handle in a laboratory. You treat this as you would treat your girlfriend/boyfriend, i.e. with care and respect.