A lid (2) for a container (67), the lid (2) comprising a first plastic layer (15) including a portion having at least one colored pigment and a second plastic layer (14) including at least one thermochromic substance capable of becoming translucent upon reaching a predetermined temperature in response to heat transfer from or to contents of the container so as to reveal the colored pigment of the first layer. Alternatively, there is described a lid comprising a layer with a portion (62,63) that becomes either observable or non-observable at the predetermined temperature.