Flexible PCB Coverlay vs PCB Solder Mask

Though coverlay and solder mask both insulate and protect the circuit, they are made of different materials and have different characteristics.

PI and PET coverlay is made of insulating resin, which is non-photosensitive. While PCB solder mask is made of UV photosensitive ink.
Flexible PCB coverlay can be bent without harm, while PCB solder mask has poor flexibility.
The color of the PCB solder mask can be green, black, white, red, blue, yellow, pink, purple, etc. While the typical color of flexible PCB coverlay is yellow, black, or white.