Some Asters seed themselves in the garden, and the seedlings that spring up won't bloom the same color as their parents. There isn't anything you can do with soil pH or fertilizer that will change their bloom color. If you don't like the faded colors, remove the plants. Hopefully, your original dark purple plant is still in the garden, hidden among the seedling plants. If you find and leave the original aster, deadhead it or cut it clear to the ground after flowering each year to prevent seeding and more disappointing offspring in your garden.