Why are Reserve cards colored? I understand the type is necessary to make them capable of calling themselves into play, but the other colored types are for ease of separation and I see no reason that Reserve cards need to be easy to identify from other cards.
The color reminds you to put the card on the mat when you play it. It's a thing players are not used to doing and can forget.
"Ease of separation" is not really the deal. Victory cards are green to remind you that they are not exciting when in your hand; look elsewhere for answers. Treasures are yellow to remind you that they can be played in the Buy phase. Reactions are blue to remind you that they may have value at an unexpected moment that they aren't directly involved in (and Tunnel crept in there on the strength of the unexpectedness of the moment, despite being involved there). Duration cards are orange to remind you that they stay out.
Attacks meanwhile have a functional identity but don't need to call attention to themselves. At one point they had a different color, and possibly some players would prefer that. You can argue that you want to note that other players can interact with you with them, although then, why not non-Attack interaction too?
Shelters are a special case; they needed a color. They have a variety of other types and couldn't be pure the other color, because Actions do that for Reactions. You can also argue some ease of separation there, and for Ruins. It does maybe help a little in your hand to have Shelters and Ruins distinguished from (other) Actions.