7 or so years ago I needed to caulk a tub and wanted a decent color match as traditional tub caulking would have been to great a contrast.
(due to tile selection I needed a dark brown)
My solution was to use a polyurethane based exterior caulking as they offered some 24 color choices.
Rather smelly until cured but very adhesive as well as a total seal and flexible.
I recently visited the owners and was impressed as to how nice those caulk joints stood up to tests of time.
But then after all it was intended for exterior usage.
PS; The brand was Flextra
It is more adhesive and flexible than traditional silicone sealants, more like windshield adhesive.
Also noted that no mildew was present.
