Observer metamerism (OM) is one of the potential issues in HDR displays because of the required wide color gamuts and high peak luminance levels. A simulation was performed using hypothetical displays to investigate how OM in HDR displays would vary with changes in color gamuts and peak luminance levels. In this work, a robust metric, observer metamerism magnitude (OMM) is introduced, which quantifies the OM of paired displays, depending on the similarity in spectral bandwidth between them. Also, the effect of changes in peak luminance on OM was found to be small, increasing OMM by 7 ∼ 8 % when peak luminance doubles.