q-state Potts model on a 2D square lattice

What you are seeing: a generalization of the Ising model where each site holds one of $q$ colors (instead of just two spin values). Like-colored neighbors bind with energy $-J\,\delta(s_i, s_j)$; the system minimizes this energy by forming large monochromatic patches at low temperature and disordering into salt-and-pepper noise at high temperature.

The transition temperature is $T_c(q) = 1 / \ln(1 + \sqrt q)$. For $q = 2$ this reduces to the Ising model and the transition is second-order: the order parameter $M$ drops smoothly to zero at $T_c$. For $q \geq 5$ the transition becomes first-order and you see latent-heat jumps in the energy across $T_c$. The $q = 3$ and $q = 4$ cases sit in between with a marginal second-order behavior.

Below: the left panel is the current spin configuration; the right panel is the order parameter $M(t)$ trace (red horizontal: zero; yellow: current value).