= Solution
At coexistence of the two pure solid forms, their <chemical potentials> are equal and $\Delta_rG^\circ=0$. Treating the tabulated <enthalpy> and <entropy> differences as constant gives
$$
\boxed{T_{\rm tr}=\frac{\Delta_rH^\circ}{\Delta_rS^\circ}=\frac{500}{1.0}=500\,\mathrm{K}\simeq227\,{}^\circ\mathrm{C}.}
$$
Above this <temperature> the model favours the higher-entropy yellow form; below it the red form is favoured. This is a prediction from the given thermodynamic data, not a guarantee of an immediately visible colour change: kinetic trapping, particle-size effects and temperature-dependent <heat> capacities are not included. A ratio of masses of two coexisting pure solids is not the <equilibrium constant>, since their bulk solid activities are each unity.
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