Create a garden plot. Grow a plant to maturity. Harvest the plant - plot will require fertilization and turn a paler brown. Fertilize the plot. Color does not darken.
I presume that, as adding manure to a garden plot restores its fertility, all aspects of the original plot are meant to return. But the color change associated with using up the nutrients in a plot doesn't reverse when the plot is made fertile again.
This is a very small bug, but hopefully easy to fix.
The garden plots should be changing colors for you when fertilized, but perhaps we've made the color change too subtle. We'll look into this. Thanks!
What he means is that, while the colors change, it will never be as dark as when you first made it.