Perhaps it's been too much Olympics that's made me ponder the importance of colour. A person in a red, white and blue swimming cap breaks a record, and we're disappointed. A person in a green-and-gold cap scrapes in for third and we're rapt. And if you want to know the difference between gold and bronze, all you have to do is ask Leisel Jones. One is sweet, the other is clearly sour.

Each sport is also separated into its own version of the primary colours. Our swimmers are generally golden, the wrestlers are black-and-blue, and our rowers are all red-faced, sometimes with embarrassment but mostly in anger.

But back to the bags, which have greater penetration throughout the country than an Australian Idol runner-up - and are designed to last much longer. These are the bags that have become the must-have item in every home.

The pundits say we were quick at snapping up VCRs when they first came on the market, or putting a mobile phone to our ears. But that is surely nothing compared with the speed at which we've taken to collecting green bags.

The Coles group alone has flogged off about 3.5 million of the bags since they started selling them a little more than a year ago. The bags are bigger than their plastic counterparts, and somewhat more socially acceptable than those red, white and blue zip-up bags that were all the rage until homeless folk claimed them as part of their uniform.

You can keep them in the car, for those emergency shopping trips on your way home. Even Danii Minogue is a fan, and she knows a thing or two about plastic. One friend uses them to balance his home budget in what he calls the Baa Baa Black Sheep system. He limits the number of green bags he takes with him when he shops, and stops when he's got three bags full.

If you want a reusable green bag, you can buy them just about anywhere. They come in all styles and colours, as long as you want them square and green. About the only difference is which store logo you choose.

You might elect to stroll the streets of Surfers Paradise with your towel and togs in a Liquorland bag. You might carry your hiking boots home from the hinterland with a Go Green bag swinging in your hand. And you might browse the craft shops of Maleny and Montville with a Woolies bag. Although, for an environmentalist, that could present a moral dilemma.

As a replacement for plastic bags, they offer more and less. They're certainly more fashionable, when you're browsing for organic fruit on a Saturday morning in Northey Street and wanting a fashion statement that says you're as green as Kermit.

But they also have their limitations. Try picking up dog poo with a green bag and you will soon be cursing the environment, or your dog, or both. And ask yourself what are you going to use for a bin liner when you stop using plastic bags and switch to their reusable cousins. You could, of course, buy plastic bags to use as bin liners, instead of using free plastic bags, but that sort of defeats the purpose. Not only does the environment pay a price, but you do too.

Then there's the trouble of remembering to take the green bag with you when you shop. And what about identification? How do you know whose green bag is whose when everyone in the nation has one on their arm?

Limitations aside, the bags are green for a reason but maybe not the reason you suspect. The bags are based on an imported model and are part of a grand conspiracy of global domination by a group of shoppers from Ireland who are the biggest thing to come out of the Emerald Isle since Mr Potato Head hopped on a boat.

The Irish supermarket chain Superquinn started to think about alternatives to plastic bags a few years ago, when the government there introduced a bag tax. First, they went to a team of industrial designers, who came up with something that met all the requirements, except the requirement that shoppers might actually want to use them.

So, they threw those recyclable bags out (presumably into another very large recyclable bag), and went back to the drawing board. And this time, instead of going to those who claimed to know what shoppers want, they went to a group of 12 experts made up of shoppers, and the poor underpaid folk who spend their days packing shopping bags.

They followed them around the store, they turned up at their homes and watched them unpack, they probed, they questioned and they generally pestered to find out what people looked for in a bag. They found people wanted something light but sturdy, no long handles that fall into the bag when you're trying to pack it, and something that looks good.