If you are familiar with laser plastic welding you understand that the upper layer needs to be transparent to the radiation and the lower layer absorbent to the same radiation.
The majority of thermoplastics transmit common laser welding wavelengths in their natural state, but a variety of factors influence the level of transmittance, including: thickness of the plastic, fillers (glass or fiber), flame retardants and color additives.
This article will focus specifically on how color affects the process.