There's not much explaining to do for this one. Your card or cards can be any card type. If you want to expand the challenge:
-Make a card of each card type
-Make a card for each four-color combination.

Excellent challenge. I've gone to the well a couple of times before, but the idea is an ever elusive one.

I tried a first stab at the concept with Boros Spy Network.

I made Day of Darkest Shadow.

­Alliance Dragonslayer from Shards.

­Zombie Firecat

­Dune-Brood Charm based on three allied-colour pairs.

Created Violent Recantation ... and Lifespinner ... and Unquenchable Flamekin ... and Parasitic Abundance . Guess I need a now.

Talking with Jack got me to make Yore-Tiller Charm

While I wait for a RGWU idea, have also Revisit, for a sneaky answer to the challenge.

And from the same idea as alex, the double-hybrid Sneaky Bear.

And Common nongreen creature

I've added Eternal Reincarnation and Omnipresence.

Added Collaboration.

­Ink-Treader Borderpost

­Physical Superiority

­Mirrodin Quisling using r/w for battle cry and b/g for infect.

­Alive // Kicking, a split card with each half a hybrid mana. It would be nicer to make each half a gold effect, but I wasn't sure what effect to use.

­Blightning Kiss four simple effects attempting to form a coherent whole.

­Four-bolt, a red-black-white-green direct damage spell. It correctly leaves out blue, but otherwise I found it hard to get a flavour.

­Phantasmal Defences, "Return all attacking creatures target player controls to their owner's hand, then that player discards that many cards at random."

­Leonin Conscription to finish my cycle of five.

­Theoretical Entomologist

­Mirror-Essence Nephlim

I forgot to link to Wrench out of Time

Oh, we appear to have a second wave of Challenge #64 going on. So I added False Oracle.

I made Terror of the Underdark and Birth of Understanding, which also satisfy Challenge # 005 as per Challenge # 073.