Gold vs. Hybrid
When designing multicolor cards, be sure that you are using gold and hybrid correctly with regards to the color pie.
Gold cards require two or more colors to cast, whilst hybrid cards can use either of two colors to cast. Compare Drakewing Krasis, a gold card, with Slippery Bogle, a hybrid card.
In design terms, this distinction has a number of ramifications. Most importantly, a hybrid card should only have abilities that its component colors could get alone. For instance, both blue and green can have a creature with hexproof, so Slippery Bogle can be cast using only blue mana or only green mana. By contrast, Drakewing Krasis has flying (an ability unavailable to green) and trample (an ability unavailable to blue), but because the card requires both colors of mana it can have both abilities.
In addition, note that hybrid cards are generally easier to cast, so should be costed with that in mind.