Seems great! DC 20? That's a pretty common number for this sort of thing.
Hmm. The original only says the difference is easily distinguishable within 30 feet to a "trained eye", but beyond 120 feet they appear identical.
Let's see, that suggests someone with a decent bonus in Spot has a reasonable chance of noticing the difference within 30 feet (a -3 Spot penalty), but none within 120 feet (a -12 Spot penalty).
A base DC of 20 would become 32 at a 120 foot range, so you'd need a +12 Spot to notice a difference on a roll of 20.
The question is what should we use for a "trained eye". It might be as little as a +8 Spot, which is exceptionally observant for a regular joe in 3E (e.g. a 1st level human character with maxed out Spot, Alertness and Wis 14), but is merely average for a mid-level adventurer.
If the latter, a DC as low as 15 might be enough, so a trained low-level character has a decent chance of noticing they're not a gold dragon.
That's assuming we want low-level characters to have such a chance, bearing in mind it'd make it trivial for high-level PCs to notice the "gold" dragon isn't such a beast.
Maybe we could make it level-dependent? Something like DC = dragon's CR. I can see an argument for that, since their coloration changes with age. That'd require changing the last sentence to something like "Noticing the difference requires a Spot or Knowledge (arcana) check against a DC equal to the Mystaran amber dragon's Challenge Rating."