Something which is mellow is soft, rich and free from harshness, particularly where our senses are involved. If we're mellow, we're either softened by age and experience - or by alcohol! (A nice mellow shiraz, perhaps?)
When describing fruit it meand soft, sweet and juicy. Mellow soil is rich and loamy.
I took this picture of my and Gary's shadows in the mellow evening light. (That's not snow but chalk - the light breeze was mellow too.) I *may* have drunk some wine when we returned to the van after our walk.