Become the Cup (Yellow)

Red Yellow Blue

As you play this, choose a color. This gets the chosen color.

Go again

Rulings

From the Compendium of Rathe release notes

Become the Cup’s first ability is a static ability that becomes functional while playing it. It is not a triggered ability. While announcing that you are playing Become the Cup, you will choose a color and it will gain that color, in addition to its existing colors. Then as normal, you will apply effects to the card that matches the effects’ descriptions. For example, if you resolve Prismatic Leyline then play Become the Cup (red) and choose blue, that card will be both red and blue for the purpose of applying Prismatic Leyline’s continuous effects.
Become the Cup’s ability is not functional while defending.
If you do not play Become the Cup, you do not get to choose a color and it will not subsequently gain the color.
If an effect prevents you from playing a card of a certain color, you wouldn’t be able to play Become the Cup and choose that particular color as you play it.
If you are under an effect that causes cards you own to lose colors (e.g. Blanch) and you play Become the Cup, you will still be able to choose a color as you play it and Become the Cup will get that color.

Keyword notes

Go again - Go again allows multiple actions to be played in a turn when usually playing an action with use up your action point, and therefore end your turn. A card with go again gives the controller of that card or activated ability 1 action point when it resolves.

When an action with go again resolves, its controller gains 1 action point.
Multiple instances of go again on the same card or activated ability do not stack. If a card or activated ability has multiple instances of go again when it resolves its controller gains 1 action point only.
If an action with go again does not resolve, its controller does not gain 1 action point.
You cannot reveal a card with no to Belittle. The card must have a value to its power that is 3 or less. (0 is a value)