Complex sentence: Both sexes display horizontal bars (unless they are too light a color for this to show) if stressed or frightened; however, such a color change, common in females of any age, is very rare in mature males.
Split into simpler sentences: Both sexes display horizontal bars (unless they are too light a color for this to show) if stressed or frightened. However, such a color change, common in females of any age, is very rare in mature males.