Thought up some random cards:
BLUE

- Splash - Tap target Red card until this card is removed.
-

- Essence of Water - :green(T) Heal 3 HP. :purple(T) Deal to damage to your opponent.
-

- Cleansing - Heal 5 HP and remove all negative status effects.
-

- Water Elemental - 5/5 -

: gain +1/+0 until end of turn.
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RED

- Flash Fire - Remove a Green source from the game to give the owner 2 Red sources.
-

- Fire Elemental - 3/6 - Burn defending creature or player.
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YELLOW

- Bulwark (Shield) - Stop the first 2 damage every turn.
-

- Earth Elemental - 1/10 - May take damage instead of player.
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GREEN

- Sprite - 1/1 - (Flying)
-

- Life Spring - Place one card from your discard pile on the bottom of your deck.
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ORANGE

- Divert the River! - Gain control of an opponent's Blue source.
-

- Cannon -

: Deal 5 damage to target player.
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PURPLE
-

- Lightning - Deal 1 damage to target creature or player.
-

- Stun - Tap target color source stack.
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