It's only constant force for quarks in isolation. A good mental model is that between the oppositely charged particles, a string of gluons forms and pulls them

It's only constant force for quarks in isolation. A good mental model is that between the oppositely charged particles, a string of gluons forms and pulls them together with constant force. The resulting composite particle no longer has a color charge, so it does not interact as much with other particles through the strong force. Of course there are also residual forces from a color-neutral bound state to quarks which are a bit further away, but those are not nearly as strong. They are for instance responsible for holding nuclei together. The important property of the strong force here is that it interacts with itself. The field of two quarks together is not the sum of their individual fields, because the force-carrying gluons are also color charged and thus interact with each other.