How does Colonel Grangerford contrast to Pap Finn?

The short answer to the question of how Col. Grangerford contrasts with Pap Finn is "in every way possible".

More specifically, Colonel Grangerford was everything to Huck that Pap wasn't. Where Pap was a mean drunk, the colonel was a gentleman who drank, but never to excess. Pap was violent and disrespectful, the Colonel believed in being the bigger man, and commanded respect. The Colonel was neat and orderly, Pap was often found drunk in a ditch.

It is not coincidence that the boy Grangerford in the story, around Huck's age, is named "Buck". Buck has many similarities to Huck, and Buck could be seen as, essentially, what Huck could have been had he had the right role model.