What characteristic do older, longer-lived stars exhibit when located in the smoother inter-arm regions?

They are cooler and consequently appear yellower or redder compared to the blue stars in the arms.

The stellar populations found in the relatively quiescent inter-arm regions contrast sharply with those within the active spiral arms. The stars dominating these dimmer areas are typically older and have substantially longer lifespans, often having burned through the most massive phases of their lives. Like our Sun, these stars possess lower surface temperatures relative to the newborn giants in the arms. According to the principles of blackbody radiation and stellar color mapping, lower surface temperatures result in light emission peaking at longer wavelengths, causing these established stars to manifest visually as yellower or redder points of light against the background disk.