Is it a faux pas to ask for a colleague's figure in a separate but related publication?
You can always ask, then it is the choice of the author to accept or not. As for the citation, if the paper is accepted but not published, you can have a note in the citation &quot;to appear&quot;. For instance, your text could like:

Following [Alice12], we describe ... as illustrated in Fig.1.
[Alice12] Alice, paper, journal, accepted, to appear.