  Yellow hypergiants (YHGs) are often presumed to represent a transitional
post-red supergiant (RSG) phase for stars $\sim$30-40 \msun. Here we present
visual-wavelength echelle spectra of six YHG candidates in the Galactic cluster
Westerlund 1, and we compare them to known YHGs, IRC +10420 and Hen3-1979. We
find that the six YHG candidates do not exhibit any metallic emission lines,
nor do they show strong H$\alpha$ emission, and as such do not meet the
criteria necessary to be classified as YHGs. In conjunction with their moderate
luminosities of \logl = 4.7-5.4 estimated from optical/infrared photometry, we
suggest instead that they are normal yellow supergiants (YSGs) with more modest
initial masses around 15-20 \msun. This adds additional support to the
hypothesis that Wd1 is a multi-age cluster with an older age than previously
assumed, and is not a $\sim$5 Myr old cluster caught at a very specific
transitional point when single-star evolution might yield Wolf-Rayet stars,
luminous blue variables (LBVs), RSGs, and YHGs in the same cluster.
Nevertheless, the population of YSGs in Wd1 is very unusual, with YSGs
outnumbering RSGs, but with both spanning a large luminosity range. Here, we
discuss evolutionary scenarios that might have led to the high fraction of
YSGs. The number of YSGs and their significant luminosity spread cannot be
explained by simple population synthesis models with single or binary stars.
Even with multiple ages or a large age spread, the high YSG/RSG ratio remains
problematic. We suggest instead that the objects may experience a prolonged YSG
phase due to evolution in triple systems.
