We review our investigations of the optical properties of β-GaN and β-(In,Ga)N layers grown on GaAs(001) by plasma-assisted molecular beam epitaxy. For In contents up to 17%, the band gap of β-(In,Ga)N layers, estimated by combining photoluminescence with spectroscopic ellipsometry as well as transmittance spectroscopy, shows good agreement with theoretical predictions from the literature. Using the theoretically predicted band gaps, we calculate 2.75eV isoenergy contours for both β-(In,Ga)N/GaN and α-(In,Ga)N/GaN quantum wells. Comparing these contours to calculations of the critical thickness, it becomes evident that blue-emitting α-(In,Ga)N/GaN quantum wells are above or at the border to plastic relaxation except for very thin quantum wells of high In content. In contrast, blue-emitting β-(In,Ga)N/GaN quantum wells lie below this border for a wide range of thickness and In content.