There are a small handful of cards that every deck in MLP CCG can benefit from.
All of these cards have 0-requirements and thus don't really matter when it comes to the deck's Primary, Secondary, (Tertiary etc) colours.
This first is from Premiere Series.
Forest Owl, Novice Assistant is an incredibly cheap card (1 AT) and in combination with other friends becomes a powerhouse!
Every and any deck should carry at least 1 Forest Owl!
Another fantastic card is the Crystal Games' Rainbowfied.
Once played and attached to a Friend, this card gives you the ability to meet and play any colour requirement. Play this Resource onto a Friend with high power and you could potentially meet any requirement!
Premiere Series' Lady Justice, Judge and Jury (again, another Friend with no requirements) is power at first glance. All opponent's Friends at this problem suffer a -1 power during faceoffs. However, Lady Justice's ability also includes your opponent's Troublemaker faceoffs as well!
This is a brutally powerful card. (Possibly even broken!)
And last but certainly not lease, we have Cloudchaser, Flexible Flier, from series 1.
Yet again another 0-requirement Friend. Her ability to allow your next Friend's cost to be reduced by 1 AT can let you play cheap Friends (ie Forest Owl) for free.
The downside of Cloudchaser is that this card is difficult to acquire, being Rare.
These card can do nothing but compliment any existing deck!