How about: (assumes rails and haml)

before_filter :find_tweet, :only => [:agree, :disagree_count]

def find_tweet
   @tweet = tweet.find(params[:id])
end

def agree
   @tweet.agree_count += 1
   render :partial => 'consensus_agree'
end

def disagree
   @tweet.disagree_count += 1
   render :partial => 'consensus_agree'
end

--------- _consensus_agree.haml ---------------

%span{:style => 'color: green'}= @tweet.agree_count - 1
%span{:style => 'color: red'}= @tweet.disagree_count

-----------------------------------------------

Also: You might have forgotten a @tweet.save there. And.. you really 
should be using classes, not styles. </nitpick>

regards,
kaspar