"NO BOYS ALLOWED!"

Posting this sign, scribbled in thick crayon, may have been the last time you decorated a door. And how many years ago was that? While walls and windows get a lot of attention -- in the form of paint, paper, and drapes -- an unexpected opportunity is, ahem, knocking. Adorn the door! "It's a chance to do something little and have something big come out of it," Martha Stewart Living's Kevin Sharkey says.

Indeed, in one bold but affordable gesture, you can enhance a room and play down a door's hollow-core blandness. With the addition of wallpaper or applique, a door can become a work of art. A door is also the perfect spot to try a daring color in a low-commitment way. (Plus, because it's such a small-scale change, it's renter-friendly).

The five projects here require just a few readily available materials and some surprisingly simple techniques. All these looks work on just about any door. The options, really, are wide open.

Play up a room's dominant pattern by painting shapes on the doors. The design of these pocket doors takes its cue from the chair backs, while the colors echo the dresser and the upholstery.

Paint, in Thunderhead (doors) and Cumulus Cloud (trim), by Martha Stewart Living Paint; homedepot.com/marthastewart. Paint, in All White (octagon); farrow-ball.com.

The beauty of this project is in its simplicity: The thickness of the artist's tape will determine the thickness of your lines. You'll use artist's tape to lay out your design; border it with painters' tape to create a stencil; and then remove the artist's tape and paint. For the paint, pick up on the colors of an accent piece (a lamp, a pillow) or a piece of artwork.

Paint, in Bayou (door), by Martha Stewart Living Paint, from homedepot.com/marthastewart. Paint, in Rich Coral (trim), benjaminmoore.com. Paint, in Spiney Green (trim), finepaintsofeurope.com. Paint, in Fig (door), and Precious Metals Brown Zircon (trim) by Martha Stewart Living Paint; a href="http://www6.homedepot.com/marthastewart/index.html?cm_mmc=Vanity-_-Martha_Stewart_Living-_-Sept10" target="_blank">homedepot.com/marthastewart. Paint, in Green Blue (door); farrow-ball.com. Paint, in San Jose Blue (trim); benjaminmoore.com.

Just as nail-head trim gives furnishings a tailored look, brass tacks can turn even the humblest door into something special. Against a door painted a high-gloss deep, rich hue that looks like leather, the rectangles-within-a-rectangle pattern mimics classic upholstery techniques and a much pricier frame-and-panel door.

Brass-plated round tacks (BS912), $68 for 50, and brass matte square tacks (BT580), $64 for 50; dadsnails.com. Paint, in Rainforest (door), by Martha Stewart Living Paint; homedepot.com/marthastewart.

Yes, a $20 hollow-core door can be made over to look like the stately entrance to an old English club. Commonly used on furniture, this trim is available as individual tacks or in continuous strips, which are less time-consuming to apply (but more limited in selection). Choose a tone and finish that are similar to the other hardware in the room. Then plot out your pattern; a simple rectangular border or tracks around the knob are good first-time projects.

Pewter high-dome tacks (BD1295), $21 for 500; dadsnails.com. Hardware knob (similar to shown), from $200; nanz.com. Paint, in Red Earth; farrow-ball.com. Brass matte square tacks (BT580), $65 for 50; dadsnails.com. Paint, in Off-Black; farrow-ball.com. Group of tacks, from top: Glazed-pewter pyramid nail (BD61-87), $42 for 60; brass-plated dome tacks (BS412), $69 for 500; and bronze high-dome tack (BR610), $36 for 1,000; dadsnails.com. Nail-head trim #5 (strip), in French Natural, $48.33 for 50 yds., and nail-head trim #5 (strip), in Nickel, $61 for 50 yds.; diyupholsterysupply.com.

There is something about a large center doorknob that suggests opulence ("The north wing and guest quarters through here!"), which is what can make it a pretty and playful addition to a small space. (So what if the north wing is just a coat closet?) Paint the rosette -- actually a repurposed ceiling medallion -- and the door the same deep color. Since a center knob like this doesn't latch, you will also need catch-roller hardware.

Rotunda ceiling medallion, 18 inches, $66; focalpointproducts.com. Oversize vintage Edwardian knob, $225; stevensclaroff.com. Paint, in Wrought Iron (door), by Martha Stewart Living Paint; homedepot.com/marthastewart.

In the hardware store, these composite plastic medallions can seem flimsy or unremarkable. But once painted and applied to a door, they look downright sculptural. Try a style that complements your space (for example, an ornate pattern for a traditional home or a sleek sunburst for a more contemporary one).

Sunburst ceiling medallion, 24 inches, $94.60, and Rondel ceiling medallion, 18 inches, $70.60; focalpointproducts.com.

Without embellishment, bifold closet doors can make a room feel like Carol and Mike Brady's boudoir. But strips of wallpaper fix that and make them look like a folding screen when ajar, or like double doors, thanks to the addition of purely cosmetic extra knobs.

Ottoman Flower wallpaper, in Mica, $94 per roll, by Schumacher; Designer on Call, 617-449-5506. Paint, in New White (wall and trim); farrow-ball.com. Mother-of-pearl knobs, $14 each; anthropologie.com.

When choosing a pattern that fits nicely inside your door panel, width is key. Look for allover patterns in small-scale geometric prints; organic patterns, such as marbling, that can be cut anywhere; or larger patterns that work within the panel dimensions.

Nitik II Wallpaper, Orange on Tint, $95 per roll, by Alan Campbell, Quadrille/China Seas; Design Professionals, 212-759-6894. Paint, in Ramie; finepaintsofeurope.com. Spanish Moss knob, in Green, $8; anthropologie.com. Villiers Stripe paper (rolled), by Nureyev, $115 per roll; Zoffany, 212-319-7220. Onyx Collection wallpaper, in Travertino, $89 per roll, by Osborne & Little; Design Professionals, 212-759-6894. Paint, in Opal Essence; benjaminmoore.com. Gemstone knob, in Clear, $8; anthropologie.com. Fern Tree wallpaper, in Graphite, $68 per roll, by Schumacher; Designer on Call, 617-449-5506. Paint, in Tailor's Chalk, by Martha Stewart Living Paint, and button knob in Polished Nickel, $3.50, by Martha Stewart Living Classic; homedepot.com/marthastewart.

Like lipstick, just a swipe of bright paint -- in this case, on the edge of the door -- makes a huge impact. And if you don't have a lot of time, this may be the project for you. All you need is paint, a brush, and painters' tape to cut clean edges. The result is graphic, whimsical, and easily reversible should you decide to change the room's color scheme.

Paint, in Bone Folder (door), by Martha Stewart Living Paint; homedepot.com/marthastewart. Paint, in Jeweled Peach (door edge) and Beigewood (wall and trim); benjaminmoore.com.

In choosing a hue for the edge of the door, find a bright accent color somewhere in your interior and amplify it a few times; wild colors you wouldn't paint an entire room work in small measure. A vivid pink or citrus stripe is also easily tempered by a neutral base color such as gray, or a deep shade such as indigo.

Swatches (for doors), from top Paint, in Cement Gray, by Martha Stewart Living Paints; homedepot.com/marthastewart. Paint, in Hudson Bay; benjaminmoore.com. Paint, in Elephants Breath; farrow-ball.com. Sticks (for accents), from top Paint, in Hot Lips; benjaminmoore.com. Paint, in Mimosa, by Martha Stewart Living Paint; homedepot.com/marthastewart.com. Paint, in Melon; behr.com.