When you have a bunch of mana floating, some colorless and some colored, and then you cast a spell, colorless mana should always be spent first by default. However, the game is not doing that currently.

This often comes into play when casting Scapeshift. For example: I float BBGG and 1 colorless mana (5 total). I then cast Scapeshift (which costs 2GG). The game should leave 1 B mana in my pool. However, the game instead leaves 1 colorless mana. Since black mana is better than colorless mana in almost every situation, this is not what the default behavior should be.

The game engine should spend as much colorless mana as possible, and only then should it dip into colored mana in your pool.