What he stood for changed over time, but at one point he said he hoped the decisions people made about his children would be made based on his children's character and not the color of their skin.

Yes he did say that one line in one speech. But if you review the totality of his writing, it is very clear that total colorblindness in the face of inequality was not his goal. The "I Have A Dream" does not represent some shift away from an earlier opinion. It simply doesn't represent the opinion that a lot of reactionaries think it does.

There's a long-running joke about MLK giving exactly one speech and never saying anything else.

Not even just one speech. Literally just one line!

So Affirmative Action is revenge discrimination?

No. MLK considered it to be a step on the path to equality.