I presume because both Spain's kits clash with Croatia's red and white, and Spain will have changed out of red, as I'm sure I once read that in international competitions one team must wear a light kit and one a dark kit if possible for the benefit of people watching on black and white tellies, which apparently quite a few people in South America still have! I swear I haven't made this up, bonkers as it sounds!
But south America isn't in Europe. .. Oh. Erm sorry wrong end of the stick To much euro debate has screwed my thinking lol