In Hillsborough County, outdoor humidity sits high for most of the year, and an interior bathroom with a small fan never really dries out between showers. Mildew does not need dirt to grow, only moisture and a surface, which is why a bathroom that was scrubbed on Saturday can look dull again by the following weekend.
The single highest-return fix is running the exhaust fan for a full twenty minutes after the last shower of the day, not the two minutes it takes to towel off. If the fan is loud, weak, or vents into the attic instead of outside, it is not doing the job. Hold a square of toilet paper against the grille: if it does not stick, the fan is decorative.
Second, squeegee or towel the shower walls and glass after use. It takes thirty seconds and removes most of the water that would otherwise sit in grout lines and on silicone. Homes where someone does this consistently need deep bathroom work about half as often as homes where nobody does.
Third, keep the shower curtain or door open when the room is not in use, and pull bath mats up to dry rather than leaving them flat on tile. Trapped moisture under a mat is where a surprising number of dark grout complaints actually start.
When it has already gone too far, cleaning products alone will not restore it. Grout that has absorbed mildew needs to be scrubbed mechanically, rinsed, dried, and then ideally sealed. That is a deep-cleaning task, not a wipe-down, and once it is done the maintenance habits above are what keep it from coming straight back.