The back of the plate doesn't print the back of the card, these must be packaged with the front plate and the back plate back-to-back in the holder, otherwise these were made special for this set and did not actually print the cards.
The upper left plate is the Magenta. Ron's hair (the reddest feature on the card) is darkest on that one.
The upper right one is the Black. The black background of the "Harry Potter" logo appears on that card and no others.
The lower left plate is the Cyan. The green stripe at the bottom of the card has the most blue in it, and this plate is the only one where that feature appears dark.
The lower right plate is Yellow. Process of elimination.
Ted Musgrave card collection 98.9% Complete: Cards Known: 1013, Cards Owned: 1002
I prefer the company of people who disagree with me for the right reasons over the company of those who agree with me for the wrong reasons.