Most math problems are not math problems. They are nuance problems.

Always shady.

 

The '8' and the 'green' cards.

Spoiler

The statement makes NO claim about the color on the opposite side of an odd numbered card, nor about the number on the opposite side of a blue colored card. Hence

You need to turn over the '8' because if it's opposite side is NOT blue, then the claim is false.

You also need to turn over the green card because if it's opposite side is an even number, then the claim is false.