Wow ! I *really* love the idea. It's the shield all Entropy Elementals have ever been wanting
A few thoughts:
A random chance to reflect spells is way too evil; therefore Jade Shield and Mirror Shield should not be part of the "possible shield effects" list. Furthermore, the description suggests that random shield effects are only applied to attacking creatures, in which case Jade Shield and Mirror Shield are nothing but vanilla shields, making it useless to include them.
About Bone Wall and Hope: both of them aren't real shields (just look at the illustrations) and both of them need a fuel that will most likely not be present in a Prismatic Field deck. Imitating Hope would be useless, and imitating Bone Wall would destroy the Prismatic Shield (unless you get free unlimited charges, which would be OP). So Bone Wall and Hope shouldn't be imitated either.
Wings: not a real shield either, and removing it from the possible effects would leave exactly one shield of each element to imitate, which sounds nifty to me.
The remaining 12 shields have an average casting cost of 4.75 unupgraded and 4 upgraded. This makes the 5 | 4 cost of Prismatic Shield perfectly balanced imo.
Let's dream: you have an upgraded prismatic shield on, and opponent's side is filled with 23 malignant cells. On an average turn, you take 5 damage (allowing you to stall with a single SoG) ; 2 cells are killed by a fire shield effect and immediately replaced as the next cell attacks (unless the last cell died) ; you gain 2 light quantas, but loose 2 random quantas ; and with the buff of skull buckler, one malignant cell will turn into a skeleton. These numbers are only theoretical means, and are slighlty rounded up.
I would give you Karma for this wonderful card idea, but I have no clue how this karma system works