The correct answer for this question is that in both of the cases the color change is occuring in the reagent added (either Bromine or KMnO4), and not the alkene.
The addition reaction of bromine with an alkene results in the production of a vicinal dibromide. Bromine has a color of red liquid, when all of this substance is consumed in the solution, the red color disappears.
Meanwhile, the KMnO4 produces a vicinal diol with reaction of alkene. KMnO4 is purple in color, but is then reduced to MnO2. This chemical MnO2 does not have a strong purple color therefore resulting in a change in color.