I also had the problem of having 3 slightly different color LED Blocks in a Stern Red LED DMD - the problem is that Stern's supplier uses whatever 8x8 LED Blocks they have laying around whether they are from the same manufacturing lot or not.
Stern said UNLESS a full LED Block was bad, a line was out, locked-on or 1 of the Blocks had a burned out pixel that they didn't consider it a defective/bad Display & they would absolutely not replace it, even after seeing a photo Pat P. admitted that while it looked bad that was Stern's policy:
"we won't replace it unless something was actually broken".
X-Pin had told me about this issue in the past as the 8x8 LED Blocks are made in odd size/number lots.
On the Stern Red DMD two of the different colored LED Blocks are on the bottom row where you can see that they are from 2 different manufacturing lots.
IIRC Brett said he can get 8 and a half LED DMD's per manufactured lot of 8x8 LED Blocks. Each DMD is 64 LED Blocks (4 Blocks tall by 16 Blocks wide) so he basically has to throw away half a Displays worth of LED Blocks (32 Blocks) if he wants them to match colors as well as possible (and he DOES ;-}).
Maybe contact Brett & see if he'll sell some of the left-over LED Blocks?
This is assuming they are the same type Stern uses - Common Anode