  In charmless nonleptonic B decays to \pi\pi or \rho\rho, the "color allowed"
and "color suppressed" tree amplitudes can be studied in a systematic expansion
in \alphas(mb) and \Lambda/mb. At leading order in this expansion their
relative strong phase vanishes. The implications of this prediction are
obscured by penguin contributions. We propose to use this prediction to test
the relative importance of the various penguin amplitudes using experimental
data. The present B->\pi\pi data suggest that there are large corrections to
the heavy quark limit, which can be due to power corrections to the tree
amplitudes, large up-quark penguin amplitude, or enhanced weak annihilation.
Because the penguin contributions are smaller, the heavy quark limit is more
consistent with the B->\rho\rho data, and its implications may become important
for the extraction of \alpha from this mode in the future.
