Board Game Arena

Under what circumstances do you have to discard a card of a particular color to buy another card of the same color? All I could find in the rulse is:

When a player buys a trading card or places a trading card from his hand in his play area, he must do the following:
He must displace an already placed card of the same color from his play area. He cannot simply place a trading card in his play area as he does the other cards. He places the displaced card on the discard space.

That makes it sound ike you can only have one card of each color in play, but I know that's not right.

The cards that come from the 4th mixed deck - when you play them it overrides the a card of the same colour and you must pay the difference.

They all have a square around the cost.

So it's just from the fourth deck. Thanks!