More info about acrylic inlays from our tech advisor Erick Coleman:
I first noticed acrylic inlays on Gibson guitars in the late '90s and early '00s, as an alternative to celluloid. Now they appear on most models except for the high-end Customs. The backside of an acrylic inlay can be painted or stained for a tinted look. These inlay sets were measured directly from a 1959 Gibson® Les Paul®.