We get questions of this nature frequently, and Adam is correct. The simple answer is yes, that is about as good as it will get.
Carpet is like a magnet when it comes to dirt, particularly when it is ground in such as with a deeply soiled carpet, or in high traffic areas. Dirt goes in easy, but is very difficult to get back out, which is why your rinse water is still murky.
The type of cleaning equipment being used and the brand of cleaning solution will also make a difference in how clean your carpets will be. For light to medium soiled carpets, your Bissell ProHeat is a good machine, as is the Hoover SteamVac. For heavily soiled and high traffic areas you need a more serious machine like the Rug Doctor Mighty Pro, or a professional cleaning service.
For cleaning solutions we recommend the Rug Doctor brand as being the most effective and economical cleaning products.