You need to first check the copyright information (you have probably signed) from the journal/publisher where the figure is published. This will determine where the copyright for the typeset article lies. The figure is art and as such should be covered by an intellectual property, which then clearly belongs to your colleague. This issue seems to be overlooked by many copyright notices. As you probably know many publishers allow authors to post submitted (unedited) manuscripts to be posted on web-sites and repositories, but not the final typeset version. This is because the journal really does not own the intellectual thoughts in a paper but the right to publish and distribute the final version.
To me this becomes quite confused and it is not clear if all publishers have considered the true ownership of figures. It would be useful to survey the copyrights to see how these issues are dealt with and if, for example, country specific rules apply.
Anyway, to answer your question: tell your colleague of your intention. It is good etiquette regardless of whether or not it is necessary. When it comes to copyright of a journal: better safe than sorry; get permission. You of course need to reference your other publication and any permission that is needed. You should add your colleague's name to the acknowledgement as the originator of the figure; again etiquette.