Color card are special cards that are very rare. They are used to recolor your rides, restaurants, shops, and coasters. Each color requires a certain level to unlock them and can drop just once. You can't upgrade them, because they don't have stats. They are purely for customization!
3 different colors can be used on a single building. Once you own a color card, you can use it on any building that allows re-coloring.
There are classic colors than can be found at any time, and special ones for every season that will drop only during that seasonal event.
You can get them in card packs from rewards, daily and free packs and card packs bought from the Shop. You can also acquire all the classic colors (but not the seasonal ones) in the Shop, regardless of their unlock level.
Visit the Build menu, in the Decoration category, Color tab to see your current color collection. Have fun and be creative!