Still new to the game, but I have several questions that I can’t get straight answers from Google.
The blues and yellows that drop with greater affixes:
For legendary or uniques:
Thank you to anyone who helps with these questions. I appreciate it
Every affix CAN be GA. Blues can drop with 1-2 affixes and yellows can drop with 3-4 affixes.
I actually haven’t used the upgrade to legendary recipe. I believe it’s primarily intended to be used for fishing up needed aspects.
You can add affixes to whites/blues/yellows up to 4… but not greater affixes. The point is that you can complete an item that has any useful greater affixes.
If you reroll greater affixes to a different affix, they will no longer be GA. However, if you reroll a GA at the occultist and don’t change it, you can lock it so it can’t be affected by the cube.
MW doesn’t add a greater affix. It enhances whatever affix it lands on… which means it can target GA or non-GA.
Basically the true best slot items are transfigured and do not brick to unmodifiable. Meaning you get the extra bit of help on the item but you can still add/change things about it.
Weapons are the big DPS as you can hit with a miracle gem damage bonus and still keep it modifiable. Has never happened yet for me. My one gem damage bonus bricked the weapon. 
Amulets you can add a second legendary power and keep it modifiable using a prism.
Blue weapons have 2 or fewer affixes. Yellow have 3-4. So you basically take those that have good greater affixes on them already…add another affix to make it 4…upgrade to legendary…then hope you get the holy grail on the transfiguration.
Not fun as I did not install this game to sit in town and craft.
Important note: only adds utility aspects that are available to your class
Just to add to this, upgrading to Legendary is exactly as Hex says here. You would only ever use it on a Rare item you didn’t typically want to use, in hopes you would get an Aspect you want. The category of the aspect can be dictated by the Prism you use within the cube when you convert it to a legendary item. Obviously it’ll be random what aspect you get within that category.
On the bright side, weapons can only ever be offensive aspects, so don’t bother wasting a prism on offensive aspects. Just throw a weapon into the cube and hit the button for those. Everything else you’ll want to use prism though.
nice info, did not knew it. not sure, if it’s so useful, but still very nice 
This is all very helpful, thank you so much. I have another question.
I have an item with a GA of Critical Strike Damage Multiplier (63%), if I Temper an item with Offensive Critical Strike Damage 60-100%, what happens?
Does it rewrite the 63% and replace it with the 60-100 or does it add 60-100 to the 63%? Very confused. I guess this goes for Tempering in general.
I could be wrong, but i think prisms only influence affixes. I don’t think you would use a prism to influence the aspect when upgrading a rare to legendary… that would simply be dictated by the type of item.
Unless i missed that detail when reading the prisms…
Totally unrelated. When you temper, you’re adding a new affix to the item. It will never be combined with an existing affix.
For example, i had a pair of boots with 2 movement speed affixes.
Additionally, you’re looking at 2 different affixes, that look a little similar.
That 63% native affix is a critical strike damage multiplier. The 60-100% critical strike damage temper is additive damage. They affect your damage in different ways. Macrobioboi recently did a video on damage that really helps demystify some stuff.
Correct, prisms only affect affixes. Aspects are random based on your class and what the item will take. So for weapons you do want prisms to avoid the useless affixes.