According to 'MARCA', the Spanish side have already decided what colour shirt to wear: purple. The club prefers this colour to the black third kit, despite using the latter in their last two away games.
History is behind that, with Real Madrid always having a link with the colour purple. That is why they have wanted the players two dress in purple for a day that could be huge in the history of the club.
In the last Champions League final between Real Madrid and Juventus, back in 1998, UEFA allowed the teams to play in their home kits, but the times have changed and everybody has to adapt.