Acid is generally the best energy type to choose as not as many things are resistant to acid (beware of "cute" DMs throwing tons of acid resistant critters at the party though). So that's a point in favor of copper.
OTOH, there aren't as many core spells that do acid damage.
If you're more into RP, check out the general personalities of the various chromatic dragons and see if one matches the way you want to play your character more than the others. Brass tend to be chatty and into gossip, bronze love the sea and seem genuinely helpful, copper are witty and love riddles and jokes, gold are noble and upstanding, silver are also genuinely helpful.
I tried to double-check my source, since it's been a while since I read it; however, I couldn't find it, so please excuse me if this information is a little outdated.
The guide I remember reading listed all the different dragon colors and had evaluated each one based on their element and the spread of the breath attack. I remember Gold being one of the ones it listed as "best" and I think Red was the other one.
However, Fire damage is one of the most common energy types, so if the enemies you're fighting are as smart as the players should be, any flame-type breath weapon is gonna get a chunk of it blocked because of Fire resistance. Bronze is a CG dragon type, isn't it? If you're looking to be CG, I'd probably go Bronze.
No, you can turn into any Dragon you want with the Form of Dragon spells. Your personal type only matters for your Arcana and bloodline powers.
When you pick a type of dragon, there are two things you want to look at:
1) What type of energy are they tied to
2) What shape is their breath weapon
#2 is noteworthy because cones are significantly more useful than lines for hitting multiple targets (especially when you can fly). Of course, you only get to use your breath weapon once per day for the vast majority of your Sorcerer career, so it's not that big a deal. Essentially, I'd say that it means Gold is better for Fire than Brass is.
#1 is important because you will need to make a difficult decision/balancing act. You get a damage bonus with spells that deal the correct sort of damage, plus, you get resistance (and eventually immunity) to the same type. In that way, a more common energy type is more useful, especially because, in a core only game, there's no way to change the energy type of a spell besides begging your GM to let you create a custom "Acidball" or whatever.
However, the other side of it is that a more common energy type means that more enemies will be resistant/immune to that type of energy. It's a tricky balance beam to walk.
For what it's worth, when it comes to how common each sort of damage is for spells/enemy damage/enemy resistance:
Fire > Cold > Lightning > Acid
So, if you take Copper as your dragon, you'll have to rely on less common spells like Corrosive Touch, Melf's Acid Arrow, Acid Pit, Vitriolic Mist, Acid Spray, etc. (Of course, most of those are not Core, so that's a problem if you're doing Core only). However, you might go the entire game without encountering an enemy that resists or is immune to Acid, and your Acid Resist might trigger a dozen times or less.
On the other hand, if you take Gold/Brass, you'll get bonus damage on quite a lot of spells--there's at least one Fire damage spell for every spell level, and still a ton just in Core. Resisting Fire will also be really helpful--you'll likely use it every few sessions (or more!). However, there are quite a few enemies that resist fire, so that will also come up a lot.
Regardless of what you choose, you'll probably ultimately still want a few damage spells that don't deal your Dragon's damage type--that way you'll have something to do if you encounter a heavily resistant or immune enemy. The 4th level spell Dragon's Breath is wonderful for that, since you can choose the damage type when you cast it, rather than when you learn it.
There's this feat that can change the elemental type of your spell. One strategy is to take Gold or Brass, because there are a magnitude more fire spells than other elements. Then, take that feat to change the elemental type, because the Sorcerer bonus is keyed to the original spell type.
cheers
Don't just look at damage but defense. If you choose fire sure a lot things are fire resistant but fire is also the most common. So that 5 resistance to fire at 3rd level is nice. At 9th it's 10 resistance and complete immunity at 20th.
I would suggest talking to the GM about the campaign. If you are going up against alot of undead, you may see more cold resistance than usual. If it is an ocean campaign, there are likely to be alot more elctricity using opponents. If you are up against the church in Cheliax, you may be fighting alot of devils and their immunities.
However, not knowing any of that, if the GM won't hint, or GM says it will be an even mix:
If you are staying CRB only, I would recommend Gold for cone and fire.
Cone gives you wider spread for your breath weapon, but that is not a huge deal. So if you like the RP of brass it doesn't make much difference.
Fire is the immunity most likely to be triggered and benifit you.
Since you are staying CRB only you may have a hard time finding enough cold, electricity, or acid spells to satisfy you.
However, if you really like one of the others it is still certainly possible. People use metamagic to still make good use of fewer spells in more spell level slots.
Like mpl said, make sure you take a few of some other type for when you are up against something immune to your primary element.
I support going for a fire-based metallic dragon: IMO, it's more about the resistance you gain against fire than the damage by your breath weapon you might have to forfeit due to the monsters' immunities/resistancies.
Ruyan.