If the requirements to be a true dragon are have the dragon types and have age categories, then it seems like a half-red dragon human would be a true dragon right?
[RPG] Do Half-Dragons Count as True Dragons
dnd-3.5edragons
Related Solutions
There have been various suggestions for mixing dragons and creating hybrids, from calculating average values for the hybrid (adding up the values of the parents and dividing them by two etc) through applying a colour theory of sorts (a blue and a yellow parent would produce a green offspring) and other solutions to applying unofficial templates (such as I linked here).
I'd suggest checking the links for a more thorough discussion. :)
As intelligent NPCs, dragons will have a range of personalities and motivations; however, since your trouble is with separating them from ordinary humanoid NPCs, I would recommend playing up the stereotypes a bit to add some distinction.
I will draw mostly from Draconomicon here, since the specified system is 3.5. It has some advice on roleplaying dragons, though a lot more on habitat and things like that.
Firstly, dragons have time. Unless they are acting in immediate defence of their hoard or offspring, there is hardly ever a reason to hurry, and it is better to do something right then to rush. To fill the time between, they will entertain their minds with puzzles, whether benign pursuits or malicious scheming as well as the accumulation of knowledge and treasure.
This also leads to their vanity and arrogance. This might vary a bit more, and some dragons are diplomatic enough to hide this from the creatures they interact with, but when one's life is an order of magnitude longer than a human's, it is easy to consider them lesser. An idea could be to try thinking of ants, or some other insect, compared to yourself. This is probably more prevalent with the ones playing the great game than the copper from the description.
They also will seek fortune and fame from the moment of their hatching until their deaths, unlike most humanoids who would do most of this in the adult/middle aged period of their lives. A dragon's hoard (in terms of monetary value) is it's indicator of status (along with age), and a dragon with a small holding will find itself looked down upon.
Additionally, you can add something more species specific to add a bit of personality between the dragons you do have. I can summarize a few of the canon ones for the types you listed, which you can add to taste.
Copper dragons are pranksters and riddlers, and very appreciative of humour. They are generally good-natured, but can be covetous and miserly, as well as very annoyed with anyone who does not laugh at their jokes.
Green dragons are belligerent masters of intrigue and back-biting. As well as treasure, they have insatiable lust for power and victory.
Red dragons are rapacious, greedy, and vain. So like normal dragons, but taken up to 11. They are the most obsessive treasure collectors, and will know the value and origin of every item in their hoard.
Silver dragons enjoy the company of the lesser races, and will protect those in need, though they are generally fairly hands-off unless there is some genuine need for action. They spend a lot of time in humanoid form, usually as either old men or youthful elves. They are probably closest psychologically to the lesser lived races with regard to time.
I'd suggest using these traits to help distinguish your dragons a bit more, at least until you get to having many dragon NPCs that are interacted with frequently. If you want some additional little things, you could do something like accent your speech (draconic puts the stress at the beginning of words, which could carry over into their Common speech) or adopt a mannerism or two.
For books, you can try Draconomicon, which is where most of the info above is drawn from. The monster manual has a tiny bit, though I'm assuming you've read that. There are a few third party books that also deal with dragons, though they might have some differing views compared to official Wizards products (though as DM, you can change whatever you like). For more general information, I'd look into the characterization of very long lived characters in fiction.
Best Answer
This issue is a mess. Please start with a similar discussion of the dragonwrought feat; many of the same arguments from there apply here.
Core: no
Monster Manual defines “true dragon” as just one of the Dragon-type creatures listed in the “Dragon, True” entry. Half-dragon isn’t there, so it isn’t one by the Core rules.
Draconomicon: contentious
Draconomicon claims to overrule Monster Manual on the subject of dragons (cf. the arguments about such practices vis-a-vis Rules Compendium), and provides a definition of a true dragon, specifically being that it “advances through age categories.” What that means is a subject of a great deal of argument, which is covered in greater detail in the above-linked dragonwrought question.
The long and short of it is that, RAW, half-dragons count as true dragons if their non-dragon race has age categories, and don’t if the non-dragon parent doesn’t. Yes, that’s insane. But if their parent does, then they have age categories that they progress through as they get older, and doing so grants them “more abilities” (rather literally, in the form of higher ability scores) and “greater power” (those higher ability scores mean they’re greater at some things than they were before – and as dragons, true or not, they don’t take age penalties). That meets the definition. Draconomicon never says anything about requiring the “draconic” age categories in the definition.
Races of the Dragon: yes, super-strictly
Races of the Dragon once again attempts to assert primacy on the issue of true-dragon-ness. It also includes a section on half-dragons, which states:
Here, Races of the Dragon defines half-dragons as “versions” of the full-blooded true dragons. The entry doesn’t have, for example, half-red-dragon, it has just “red dragon” and then gives stats for the red half-dragon. As such, it states that a red half-dragon is a “version” of a red dragon, and is therefore a true dragon (and, in fact, a red dragon as well).
Intent: impossible to say
The distinction between true dragon and not is only rarely relevant: one must be a true dragon to have a Dragon Psychosis from Dragon vol. 313 or a Sovereign Archetype from Dragons of Eberron. Aside from these, however, the rules never make a distinction between true dragons and other dragons.
The authors of that Dragon article and Dragons of Eberron were not the same as the authors of Monster Manual, Draconomicon, or Races of the Dragon. They may not have even discussed this issue with those authors, or been aware of the various definitions. They also did not address half-dragons or dragonwrought kobolds specifically. So who knows what they expected?