I have always wondered how the bright pigment of a plastic becomes integrated into its structure without actually harming the structure.
How can Lego blocks, for example, be a multitude of bright colours yet still retain exactly the same properties?
Generally, the dye or pigment is premixed with a polymer and then the polymer is added to the plastic itself. They don’t become part of the chemical structure of the plastic; they become a mixture. A red Lego brick is really a white Lego brick with zillions of tiny pigment molecules that make it appear red.
Why don’t these particles rub off in your hand? They do, to some extent, with paint. What is the difference?
They’re incapsulated (surrounded) by plastic. In plastics that aren’t very UV-resistant, you can get the pigment to rub off like dust when the plastic has been exposed to the Sun for a long time.
BTW, plastics start out as some neutral color - the pigments are mixed in while the object is being molded. The pigments come in little “crystals” that are mixed with the beads of plastic.
Don’t know, really. I think that the polymer which the pigment or dye is added to first is designed to adhere to the plastic- perhaps the polymers themselves do bond at the molecular level.
Some plastics are painted, rather than colored; usually via electrostatic coating or just airbrushing or whatever, and that stuff does come off.
I’m really not sure what you mean when you say “the polymer is added to the plastic itself”. It’s true that the plastic is not entirely polymer, but certainly for something as rigid as a Lego, the majority is polymer. Pigments tend to be very absorbent (spectrally) so it doesn’t take much pigment to get a good color. These quantities are not likely to effect the properties of the polymer much, although I’m sure that they do.
I guess in the case os Lego’s (Acrylonitrile butadiene, ABS), acrylonitrile and styrene are polymerized with polybutadiene. I don’t know enough about ABS to know how far polymerized the butadiene is when ABS is made.
It makes no real difference to the color though. If a pigment causes a problem with the plastic, then they pick another pigment. It is not very difficult to design a pigment if needed. As was stated, the pigment doesn’t rub off on your skin, because it is encapsulated in the polymer.
How much magnification would I need to view these colour particles?
No magnification at all, you are already seeing them. 
Of course you really wanted to view the individual pigment molecules. You can’t. Unlike the polymer, the pigments are small molecules. At that scale, the color of the molecule doesn’t mean anything. The visual appearance of the molecule doesn’t mean anything. There are ways to study things that small, but a polymer like this is too disordered to pick anything useful out.
A scanning electron microscope ought to do it.
Most of your common polymers are thermoplastics – they consist of long molecules forming a chain, bound together by chemical bonds. Many such strands are held together by somewhat weaker foces,. On a molecular scale, these plastics look like a plate of spaghetti with a severe case of static cling. When sych plastics are melted, it gives the strands enough energy to slide against each other, and so such thermoplastics can be melted and reset repeatedly into new shapes, as long as the temperature doesn’t go too high.
The dye gets incorporated into the spaces between these strands. It’s not as if the plastic was a large single crystal. There’s plenty of space to put in the dye. And not only dye – plastics often have other components added, such as plasticizers (that make them more flexible), UV absorbers and agents to take up ions, to prevent aging from UV exposure. Sometimes they have things like perfumes added, or anti-fungal agents. They’ve added fibrous material in “filled” polymers to add strength, and added metallized glass beads to improve EM shielding in computer cases. I’ve added chemicals and fibers for physical applications.
Of course, that’s not the end of it. There are block polymers and thermosetting polymers and cross-liked polymers (like vulcanized rubber), but even in these, you can put such dopants in and they’ll find spaces to reside.
And here I thought GGW videos were vile enough to put an end to crass consumerism. But putting harmless Pygmies into plastic Legos? Now that’s just sooo wrong on so many levels I’m left almost speechless.
In fact, I’m starting a Pygmies Rights Group this very minute. Please send all contributions to “RedFury4Pygmies@NigerianScams” and I’ll get right on/beneath it.
Dang! Just when I thought I couldn’t summon any more outrage, this had to happen. 
Apropos of nothing, my first website was www.angelfire.com/fl/freethemidgets .
Seems to have been deleted.
<last off-topic, promise>Interesting. But since I don’t wish to derail this thread any longer, allow me to suggest you open a thread on this very topic; perhaps we can combine efforts on behalf of these much-maligned little people. Kid you not.
Joking aside, as a rather tallish/physically normal person myself, would that we could mainstream midgets. On a personal note I do my best not to ogle any kind of abnormal physical condition. Must admit though, bill-board like tattoos and facial pin-cushions still draw my gaze; in my defense, I do see them as choices rather than congenital differences. For instance “chin-chin” Hare Krishna’s seem to always catch my eye (MIA airport quite the niche) – doubt they enjoy my GD-like approach when speaking to them.
Not that I mind really, it’s just a nod to being a 52 year-old Uber Liberal. Some things have simply passed me bye.</back on topic>