LenS expounded:

Today I picked five cucumbers and noticed that two of them were
yellow. Now, I've seen yellow cucumbers before but they've always been
scrawny and obviously not well. In the past we've picked them and
discarded them. These, however, appear healthy and mature, good sized,
and firm.

Usually yellow, good sized cukes are past their prime and seedy. Let
us know what you find when you cut into one.

Dunno about your red spot )
--
Ann, Gardening in zone 6a
Just south of Boston, MA
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