  Extending the famous Model B for the time evolution of a liquid mixture, we
derive an approximate expression for the mobility matrix that couples the
different mixture components. This approach is based on a single component
fluid with particles that are artificially grouped into separate species
labelled by ``colors''. The resulting mobility matrix depends on a single
dimensionless parameter, which can be determined efficiently from experimental
data or numerical simulations, and includes existing standard forms as special
cases. We identify two distinct mobility regimes, corresponding to collective
motion and interdiffusion, respectively, and show how they emerge from the
microscopic properties of the fluid. As a test scenario, we study the dynamics
after a thermal quench, providing a number of general relations and analytical
insights from a Gaussian theory. Specifically, for systems with two or three
components, analytical results for the time evolution of the equal time
correlation function compare well to results of Monte Carlo simulations of a
lattice gas. A rich behavior is observed, including the possibility of
transient fractionation.
