If you want a bright tomato sauce or marinara but need to thicken it up, there's only one answer. You have to cook it for longer so the excess water evaporates.

You can boil or simmer the sauce to thicken it. While boiling with the lid off will be faster, simmering will reduce the risk of burning and splattering.

Water generally evaporates at a rate of 1% to 2% per minute at a vigorous boil. How long the process takes depends on how much sauce you have and just how watery your sauce is.

As the sauce boils down, the thicker sauce will slip to the bottom and the thinner, watery sauce will float on top. By stirring, you can see if the sauce is thicker than it seems.