By default Anthem doesn't provide you with all materials for your Javelin. Some you can purchase from the store, while others you have to unlock. Accessing the gold paint finish isn't an easy task, but there are ways of replicating it far easier. 

Unlocking Gold 

There are a total of 3 Factions in Anthem:

• Arcanist
• Freelancer
• Sentinel

Each Faction has 3 levels of Loyalty, with the Arcanist and Sentinel Factions providing incremental paint finish rewards at levels 2 and 3. 

• Arcanist Loyalty 2 provides: Tungsten, Silver and Platinum
• Arcanist Loyalty 3 provides: Bronze, Brass and Copper
• Sentinel Loyalty 2 provides: Tin, Iron and Lead
• Sentinel Loyalty 3 provides: Nickel, Chromium and Titanium

When you've completed the Faction Loyalty for all 3, up to Loyalty 3, you'll unlock a new quest-line called The Champion of Tarsis. This unlocks further Faction Loyalty with each, which required a total of 50,000 to complete. In total, you'll need 150,000 Faction Loyalty in this new quest-line to unlock the Gold material. Needless to say, this is an enormous goal that for the most part, is unattainable. 

The Cheap Version

A fully decked out Ranger in Brass.

If you're wanting a gold-look quickly, the easiest method is to get Arcanist Loyalty to 3. You'll then unlock Brass, at which point you can just equipped it under any Metal base material, before selecting Brass. Not only does it look pretty much identical, but it saves you grinding a rediculous amount of Loyalty across 3 Factions.