Leaderboard question

In the competition leaderboard, under the 'challenge' column, the challenge button is usually black with white text (the one you click on if you want to challenge that driver and chase their ghost).

But can anyone tell me please, what does it mean when the challenge button is coloured purple?

Thanks

 

I think it means you haven't purchased the car you want to challenge (or the skin only).

 

ah, ok thanks