為什麼膠袋(polythene bags) 的顏色會脫落呢?

Is polyethene what you mean?

The pictures or logo are print on the the bags using printing and silking technique, thus, the adhesion of the colorant/ ink may not be strong enough to be stay attached on to the surface.

If the color you mean here is the original color of the plastic bag, it will fade away when:
1) Disposed under sunlight as the plastic begin to decomposed.
2) When you strect it. Actually the plastic chain is just deform, making the color look paler.

膠袋如果你用絲印的話是不會脫色的! (明冇)