Yep, the chance a Pokemon is a shiny Pokemon works the same way as 3rd and 4th gen games -
here's a link.
And there are no ways to increase your chances with finding a shiny Pokemon, unless you use the Pokeradar for Diamond and Pearl.
Also - there's nothing odd about the fact that you haven't encountered a shiny Pokemon thus far. In Colosseum only 'gettable' Pokemon can be shiny - in fact that's for basically every game bar for two trainers in FR/LG, and the possibility of 'renting' a shiny in Emeralds Battle Frontier.
So in Colosseum, there's 48 Shadow Pokemon + Espeon/Umbreon + Plusle + Ho-oh makes 52 pokes that could be shiny. In XD, well, let's be rather generous in your wild Pokemon encounters and say you see 200 Pokemon that you can get per run-through. Three Colo run-throughs and two XD run-throughs makes for 556 Pokes that could be shiny - around a 6.8% chance. You'll need to repeat that more than 14 times by that chance...
And that's assuming that your luck is dead-on average. Heck, there's a chance you'll never see a shiny. There's nothing to it but good old random luck. So just have paitence, and you might just get one.
You could always go for the soft-reseting option, but for Lugia it's timely and for Ho-oh nigh impossible without losing your sanity, methinks. (Saves after you get it, not when you actually can tell for sure if it's shiny).
Or if you don't care where you get a shiny Pokemon from or which one and have Diamond and Pearl, (or are getting Platinum), there's an easier method via 'chaining' by using the Pokeradar - look in the 4th gen sections or do a google search. Basically it's a technique that if honed allows you a far greater chance at finding a shiny Pokemon.