The liner is not packaging. It is the surface the die cuts against, the web the sensor sees, and the reason your labels either peel cleanly or tear.
Glassine is a supercalendered paper: dense, smooth, translucent. PET liner is a film: dimensionally stable, stronger wet, and more expensive. Both release the same adhesive; they differ in how they behave on a high-speed line.
Blue liner is a visual and optical convention. It separates thermal stock from coated stock on a warehouse shelf at a glance, and its colour gives a gap sensor a stronger contrast edge than white-on-white. On a white liner, a white label and a white background can leave the eye-mark sensor hunting.
White liner keeps the converted roll visually neutral so the printed face reads as the only colour in the pack. For cosmetic and retail work the liner is often seen during application, and a blue edge on a white bottle is a distraction the brand team will reject.
Release force. Two liners of the same colour can differ by a factor of two in peel strength. If a customer reports labels lifting at the edge or the web breaking at speed, the colour is the last thing to check and the release curve the first.
Tell us the printer model, the application speed and the label size. From those three we recommend glassine or PET, 62 or 80 gsm, and the liner colour. Guessing from colour alone is how a roll ends up unusable on a line it was never tested on.