I agree that Marianne is contradicting herself. She is so strict!

First, Lady and Sir John Middleton's raptures show shameless want of taste.

She thinks Edward has no real taste (ch .3). Edwards approbation was very far from that rapturous delight, which, in her opinion, could alone be called taste.

Her thoughts about Edward are almost repeated exactly with the Colonel in the quote you have given. Except that she allows that the Col. Brandon has some taste. She just thinks he's too feeble to have exquisite power of enjoyment.
So what's the difference between Edward's admiration and Col. Brandon's? How did she recognize Col. Brandon's taste or pleasure? Perhaps Col. Brandon's eyes have that spirit, that fire, which at one announce virtue and intelligence that Edward's eyes lack in ch.3.