Only Smarties have the answer!
Smarties first appeared in England in 1937, under the unassuming name of Chocolate Beans: it wasn't until the following year that they were renamed Smarties by their manufacturers Rowntree's of York. At the same time as coining the now-familiar brand name, Rowntree's repackaged their chocolate beans in cardboard tubes – eventually giving primary school teachers nationwide something better to use to demonstrate the shape of a cylinder than the inside of a loo roll. Their usefulness to teachers is, in fact, twofold: the plastic lid of each tube (previously either red, yellow or green, but now exclusively orange) has a letter of the alphabet embossed on its underside in lower-case, and it's common to find people collecting these handy alphabet counters for use in decoration and play.
According to the tube, Smarties are 'milk chocolate in a crisp sugar shell'. They come in eight colours: red, orange, yellow, green, blue, pink, lilac (Nestlé call it 'mauve', or sometimes 'violet') and brown (my mother spent years trying to convince me it was 'burgundy'); each Smartie is, for want of a more accessible comparison, approximately the size of one's iris. Originally, light brown Smarties were made in lieu of blue ones; the latter were introduced in 1989 to mark the 50th anniversary of the brand. It's true that orange Smarties are flavoured orange, but they're not the only ones to be singled out: until 1958, dark brown Smarties were made with plain chocolate, and light brown ones with a coffee flavouring.
There have been a number of limited production runs of 'special' Smarties since the original blue Smarties promotion, including white chocolate Smarties (which I remembered vividly as having existed in the late '80s or early '90s, but never managed to convince anyone of this), 'Cool Dude' Smarties (in which each lilac Smartie was printed with a 'cool' face, complete with sunglasses), Cola-flavoured Smarties (vile), and, so Nestlé tell me, 'Gruesome Greenies' and 'Zappy Orange' Smarties. Now, you can find Smarties in king-size tubes and boxes, chocolate bars, ice-cream (and McFlurries), and many places selling cookies make 'rainbow' cookies with Smarties baked in them. There are Mini Smarties, Giant Smarties and Smarties Mini Eggs. They make good buttons for gingerbread men, too.
At the time of (re)writing, a normal-sized tube of Smarties should cost you around 35p – not quite the 2d the original Chocolate Beans sold for.
'Smarties' is a registered trademark of Societé des Produits Nestlé S.A. – except in the United States of America, where Nestlé sold the name to Ce De Candy, who produce the (apparently far inferior) American candy of the same name. Their website is at http://www.smarties.com.
Sources:
http://www.nestle.com/html/brands/products/smarties.asp
http://www.classaxe.com/smarties
and an email from Nestlé Customer Services assuring me that white chocolate Smarties did, in fact, exist, as well as giving me a short (unrequested) history of the product.