According to this guide and ign, gold is indeed the rarest color for weapons. The ranking is as follows:
The color scheme used in Apex Legends can be a powerful tool once you get familiar with it. The colors represent item rarity with gray being the least rare, then blue, then purple, and finally gold being the rarest.
Loot dropped by eliminated players will glow the color of the rarest
item that it's in the stash, making it easier for you to decide which
stash to loot first.
There aren’t different tiers/rarities of individual weapons i.e. there aren’t grey, blue, and purple prowlers, but there are tiers for wearables and attachments.
For attachments, the higher the tier/rarity, the better it is. Clips of higher rarity hold more ammo, stabilizers of higher rarity give more recoil control, stocks of higher rarity provide faster raise/lower times when swapping weapons, etc. The only subjectivity comes with scopes where it depends on preference as higher tier scopes have more zoom and gold scopes have the added benefit of highlighting enemies in red and showing enemies through smoke.
Gold weapons are the exact same as non-gold versions, except they come fully kitted with attachments that cannot be swapped out. There are also two gold weapons (Kraber and Mastiff) which can only be found in care packages and will always be gold.
Gold wearables are essentially equivalent to their purple counterparts, except they provide an additional passive perk.
- Gold helmet: faster recharge of abilities.
- Gold armor: full shield refresh on execution of an enemy.
- Gold backpack: faster usage of healing consumables.
- Gold knockdown shield: single use self-revive.