Created a figure for a colleague's thesis, want to use that myself as well
With all due respect to your TikZ prowess (which indeed is more impressive than my own non-existent TikZ skills), these figures are... not exactly the Mona Lisa. Anyway, they are your work and you should feel free to use them and not say anything at all about the circumstances of their creation. If we had to give credit for every microscopic idea that inspired a turn of phrase or visual idea in the papers we write, they would all be three times as long and virtually unreadable. The situation you are describing is simply too trivial and unimportant to warrant mentioning in my opinion.

As for the future, if in the future you contribute a more substantial intellectual product to a project that then publishes it without an acknowledgement to you, then later want to use the same product in your own publication, then because of the concerns you mention here it may be sensible to accompany the figure or whatever it is with a footnote along the lines of "This figure is the author's original work, and was used with permission in [insert reference to earlier work that used your figure]."