Want to keep the planet and your portfolio green? The Fool shows you how in our special series on Earth-friendly investing.

Like the frog said, "It's not that easy being green."

Sure, everybody wants a piece of the green revolution. But for all of its good intentions, green technology carries with it a whole host of issues. Take hydrogen fuel cells. Companies such as Plug Power (Nasdaq: PLUG) and Ballard Power (Nasdaq: BLDP) have their corporate hearts in the right place, but their income statements are a mess. The technology isn't ready for prime time, and that's why companies such as Toyota (NYSE: TM) and Ford (NYSE: F) have chosen (lucrative) half-measures such as hybrid gas-electric engines.

And even here we find problems with being green. In the midst of a recession, consumers' spirits may be willing, but their pocketbooks are weak -- too weak, perhaps, to afford the extra upfront cost to buy a hybrid version of a standard gas-guzzler. And don't even get me started on green building, solar panels, and the like -- if the extra cost of a hybrid is a stretch, then the tens of thousands of dollars it can cost to "greenify" a home would tie Mr. Fantastic in knots.

A light bulb turns on
Commercially unviable, economically shock-inducing green tech threatens to derail the best-intentioned plans of mice and Fools. But there's at least one way to go green without going into the red -- for consumers, and for the investors who track them. Ladies and gentle-Fools, I present to you the humble light bulb -- or, rather, its 21st-century cousin, the compact fluorescent light (CFL) bulb.

Selling for as little as a couple of dollars apiece, the CFL is affordable to all. More energy-efficient than its incandescent progenitor, it also lasts eight to 15 times as long as your average bulb. And the companies who capitalize on its introduction are bound to earn beaucoup bucks from the Green Revolution.

So how do we invest in it?
Ah, there's the rub. Most CFLs are manufactured either by companies so big that CFL sales won't move the needle on their profitability -- think General Electric (NYSE: GE) -- or by private companies.

So your best hope of profiting from CFLs, it seems, is to move one step downstream and buy stock in some of the retailers who hire out the manufacture of their private-label CFL bulbs. Such retailers include Wal-Mart (NYSE: WMT) and Home Depot (NYSE: HD), which have the added virtues of being Motley Fool Inside Value picks. Like the industrial giants, their size will realistically prevent CFL sales from meaningfully boosting their earnings. However, Home Depot is parlaying CFLs into a marketing tool to attract shoppers who will then spend more on other merchandise.

It may not be easy being green, but if you're clever enough, you can still profit from it.

Fool contributor Rich Smith does not own shares of any company named above. Wal-Mart and Home Depot are Inside Value recommendations. The Motley Fool has a disclosure policy.