Your XP bar turns pink at the end of level 25 to show that you already have enough XP to get to level 26, but you’re now working towards a bonus level. This distinguishes it from your regular blue XP bar, which has reached the end of level 25 and should stay there, but hidden, until level 26 is released.

When level 26 is released:

– your XP bar should turn blue again,
– you should have a head start on level 26 XP
– the bonus levels will probably be moved to the end of level 26

For more on the bonus levels and how they work, take a look at this post: What happens at the bonus level for level 25 players?