Can someone please explain to me what is 'a good helm' in this game? ... beside looking good of course...

TLDR: When deciding which head item to pick, beside max defence, socket and which mats are needed for upgrades, are there any other concerns I should be addressing?

Hi guys, I am new to this great game and I still have a lot of things to learn so please don't stone me for asking this...

Are all head items (helms) that provide 21 defence (when fully maxed), except unique helmets (like Plate helm of the Shadows) created equal and can I pick up ANY of them and just put ... abyss... thing in the socket and call it a day?

Guide that I am following recommends using Tariv gloves / Boots / cloak and Golden Greed Plate helm. I don't like the looks of that helm and I can't see why someone put that particular helm in that build.

When I was looking into finding adequate substitution I learned (from AI so I am not 100% sure if it's correct) that unique helms (like that Shadow helm and Marni helm) can be upgraded but unlike 'regular' gear, you need to use some special (more rare mats) for upgrades. Although they do provide some nice perks I decided not to mess with those so I am looking for some other helms that can be upgraded more easily. I am interested in helms that provide max defence (21) and have 1 socket that I can use to remove / insert... Abyss...thing... 'rune'... (I'll call it 'rune' from now on, you know what I am talking about). Anyway, I saw some nice helms, some of them provide resistance to stun (but I can't which ones for the life of me at the moment).

Currently, I am looking at helms at this site and every helm I check, that has defence 21 when fully maxed, has just that one stat - defence - no stun resistance, no any other perks beside socket (again, I am not talking about unique items that are more difficult to upgrade). They don't provide bonuses to other stats, like attack, element resistance, stamina or something else. For reference, I am thinking of following helms (purely for esthetics, 21 def and socket):

- Official Knight's Plate helmet
- Lightning bolt Plate Helmet
- Scorchflame Plate Helm

Bonus question: Why different items have different colors on that site? For example, Lightning Bolt Plate Helmet is yellow (has 21 def and 1 socket) and Golden Greed Plate Helmet is orange (and has same def and 1 socket). I read somewhere about 'sets' and that Golden Greed is part of the set - is this the reason for different colors? Thanks!