Only large parts of gear (body, legs, head, shield) allow full benefit from the enchantment, in the case of other parts the enchantment is capped.
For instance, a gold and a purple glyph on your belt will have the same effect (and maybe even weaker glyphs?). But if you try with your cuirass, you wil definitely see the difference.
I am sorry I cannot give you the exact value of this cap and the optimal pairing item-color, maybe someone else can give you more details...
Caveat 3: Large pieces (head, chest, legs, shield) will have full enchantment potency. This is why people recommend using infused on them. Other pieces have somewhere around 40% the enchantment potency. So a gold enchant on a belt will give you less than a gold enchant on a helmet. But you will still see differences between the different enchantment levels, so a gold enchant on a belt will still be stronger than a purple enchant on a belt.