Please do your research before posting this, the quote was not made by Hitler but by Gregor Strasser who was killed by Hitler and many others during the "Night of the Long Knives".

If you just don't have a lot of time, you can use Hitler's own words from Mein Kampf, where the Furhrer clearly illustrates his contempt for the "leftists," and had used their colors (not to mention their name) to annoying them:

"Yes, how often did they not turn up in huge numbers, those supporters of the Red Flag, all previously instructed to smash up everything once and for all and put an end to these meetings. More often than not everything hung on a mere thread, and only the chairmans ruthless determination and the rough handling by our ushers baffled our adversaries intentions. And indeed they had every reason for being irritated.

The fact that we had chosen red as the colour for our posters sufficed to attract them to our meetings. The ordinary bourgeoisie were very shocked to see that, we had also chosen the symbolic red of Bolshevism and they regarded this as something ambiguously significant.

The suspicion was whispered in German Nationalist circles that we also were merely another variety of Marxism, perhaps even Marxists suitably disguised, or better still, Socialists. The actual difference between Socialism and Marxism still remains a mystery to these people up to this day. The charge of Marxism was conclusively proved when it was discovered that at our meetings we deliberately substituted the words Fellow-countrymen and Women for Ladies and Gentlemen and addressed each other as Party Comrade. We used to roar with laughter at these silly faint-hearted bourgeoisie and their efforts to puzzle out our origin, our intentions and our aims.

We chose red for our posters after particular and careful deliberation, our intention being to irritate the Left, so as to arouse their attention and tempt them to come to our meetings if only in order to break them up so that in this way we got a chance of talking to the people."

Sources:

http://genius.com/Adolf-hitler-chapter-7-the-conflict-with-the-red-forces-annotated

http://www.csun.edu/~vcmth00m/NazismSocialism.html