Mythic Items (Purple) are made by using a recipe of 4 Mythstones to make a Mythic Affix. there are only 2 Eternal Items in each Class that can be Crafted with a Mythic Affix, the Mythical Set Items. any other Item with 4 Sockets and the correct Mythstone Recipe can be made into a Mythic Item.
Eternal Items automatically have a +50% Item Quality and all Affixes have their values doubled, except for Crystal Affixes.
this is the only way Eternal Items can be Crafted 
one thing I didn’t mention in the Post above is that Eternal Items have a small random chance to be found with a Crystal Affix if there is already an Epic or Legend affix that is on the Crystal Affix List.
in certain builds, an Eternal Item or two could be an important part of the Build. but you can’t use most Crystals on them, which is what makes using Legend items better most of the time, as you can use all Crystals on them, except for using Quartz on the Seven Deadly Sins Set Items.