\pgfkeys{/my key/.initial=red}
% "/my key" now stores the value "red"
\pgfkeys{/my key=blue}
% "/my key" now stores the value "blue"
Note that with this configuration, writing \pgfkeys{/my key} will not have the effect you might ex-
pect (namely that blue is inserted into the main text). Rather, /my key will be promoted to /my
key=\pgfkeysnovalue and, thus, \pgfkeysnovalue will be stored in /my key.
To retrieve the value stored in a key, the handler /.get is used.