Since then he's shown his true colors by being pro-establishment.

He was a great party chair for the DNC between 2004 and 2008. He leveraged his progressive campaign into a lasting network of voters turning out in the midterms.

I don't think that he showed his "true colors" when he became a lobbyist. It's a bit more complicated than that.

Obama in 2004 was one of the "Dean Dozen" that Howard Dean endorsed for Congress. And in 2008, Obama was able to beat the Clinton machine largely because Dean was in his corner. He didn't face any of the bullshit Bernie did in 2016. Nobody from the DNC (like Donna Brazile) went to the media that year to spin pro-Clinton and anti-Obama stories. They let him have 26 whopping debates instead of just 9. They started the debates 8 months ahead of the primaries instead of just 2 months. And on top of that Obama inherited Dean's grassroots network from 2004 as well.

For all this support, Dean was reasonably expecting a cabinet position in Obama's White House. He didn't get it. My speculation is that Clinton made Dean pay for his support of Obama. She threatened to go to war within the party and managed to extract a bunch of concessions from Obama like the SoS position and the ousting of Dean as party chair. There was absolutely no reason why the DNC should have ditched such a great party chair whose 50-state plan successfully won the party three consecutive Congressional terms. But despite all reason, Howard Dean was pushed out of the position. Tim Kaine came in his place, who as we all know later stepped down for Debbie Wasserman-Schultz and eventually became Clinton's VP pick. The establishment, after failing to nominate its chosen champion, basically moved heaven and earth to make sure that it would never happen again.

At this point Dean gave up his crusade and joined what he couldn't beat. He became a lobbyist. He became part of the establishment. And he started looking down in disdain on any other progressive still fighting the fight that he abandoned.

Point being that being a lobbyist wasn't Dean's "true colors" all along. He changed over time because of his experiences. I wish he hadn't, but he did.