Now, this is a tricky one.
Black and white has the highest colour contrast
Yellow and black has the highest hue contrast
(Trivial Pursuit sides with the latter, and I should know because I've yet to be beaten. German, English, New Zealand, US - it makes no difference. This is what comes from having a pitifully small obsessive brain. I'm probably mildly autistic, actually...)
So here are two to choose from
Black. White. Old. New.
The Mainz Hyatt towering above one of the gates of Fort Malakoff, a Caponnière incorporated into the 19th century defensive bulwark along the Rhine, that gets its name from an especially solidly constructed defensive tower in Sebastopol during the Crimean war. ("Charge of the Light Brigade" etc)
"Black-Yellow. No thanks."
Highly topical (and their sentiments, not necessarily mine.)
"Black" is the centre-right CDU party of Chancellor Angela Merkel.
"Yellow" is the pro-market Free Democratic party of Guido Westerwelle.
Their policies are suspect to the centre-left SPD and an absolute anathema to the Green party, the author of this election poster.
(They won Sunday's election, anyway...)