Pick three accent colors and work with the brown.  With a deep, chocolate brown I would choose burnt orange, golden yellow, and a deep red. 

With brown walls, your dining room/kitchen furniture can be black.  Add black frames to the walls, black candles, or framed art with your chosen accent color.  Deep chocolate, black, and burnt orange looks beautiful in a dining room.

Add your accent color through accessories such as throw pillows or blankets. Baskets with multicolored liners and lamp shades also work to pick up accent colors. 

Be sure to add a large area rug to your floor to emphasize your color scheme.  A rug such as this neutral/red contemporary would go great in a brown room.

How could I incorporate sand with the beach theme I'm planning for my bedroom?

Generally sand is something I'd suggest you clean from your floor as opposed to using it for decorating, but if you want to put it in a vase to admire and remind you of days spent on the surf, I'd take three large vases in varying sizes and styles. Buy a few square vases and a cylinder. Layer each with sand and shells or fill up with sand and add a candle to the center. Keep in mind that there is different types of sand and they will look different in vases.

If you are having a beach theme in your room, there are a ton of other great ideas for stuff you can add. Add a large surfboard to your floor by choosing this area rug for a 'Surf's Up' kind of feel or this one if you feel like Riding the Wave right on your floor.

You can hang signs from the beach such as "No surfing" or "Lifeguard on duty" as decor for your room as well.

A beach room is a fun room, so use your imagination!

Find accessories that highlight your tastes.  If you like antique lanterns, find a way to work them in by using color and shape to incorporate them.  Frame your favorite photos and set up a collage or a collection on your end tables.  Use your imagination!  Decorating should be fun, not daunting.  Feel free to express yourself and you'll end up with a beautiful result.

I love offices.  They make one feel so organized when at home. You can relax and get things done in an office that is properly organized.

If you can't paint to add color, you need to add it in other ways.

Given that you have a desk you'd like in there, as well as needing a place for your cats, I'd suggest making it into a mini-office/rec room.

Place your desk and chair in the middle of the room, with the back to a a window. Be sure your desk is facing toward the door. When your desk is facing the wall, it gives a very negative view of the room. Depending on your budget, add 3 large book shelves behind your desk. You can buy different style of shelves from Ikea, and they are very economical as well as stylish.

Hang some curtains on your window. This increases the cozy factor. With a black desk (and plain walls), I'd focus on colors such as taupe, dark brown, or light cream. With no color on the walls, you can get really bold curtains that will add color to your room. With dark browns and creams, you could have a neutral curtain with orange or red accents.

On your floor, add a large area rug. There is nothing that says 'Welcome home' more than an area rug. Try a large rug such as this neutral with burnt orange/red accent such as this area rug. It will look great under your black desk.

Add a cushion to your desk chair. Even if you don't sit with it there, it looks cozy when you walk into the room.

If you want to add a bit of extra to your office, put a flat screen TV on the wall facing your desk.  Hang several picture frames around the TV, creating an almost collage of frames.  When your TV isn't in use, you can put a screen saver on it that will only add to the ambiance of the room.

Hide your cat litter boxes by removing the closet doors and hanging a curtain across the expanse of the closet.  Your cats can come and go, and the litter boxes will be hidden from view.

How can I decorate a living room with white walls? I like olive green, dark brown, and orange for color.

Nice color combo!  It would definitely look cozy with the right touches.

Depending on what type of couch color you have (chocolate brown or olive green) add a large area rug under the base of the couch, extending under your coffee table.  For a true homey touch, get a deep pile area rug so that you have something cushy to sink your feet into when getting up and down.  A Flokati rug is so soft, has a long pile and adds a touch of luxury to your room.

With white walls, you really need to refocus the eye and use the white as a back drop.  Add some color with frames and other knick knacks.  One idea is to create a collage of black or deep brown frames, blowing up your favorite photos and putting them up in a set pattern on the wall.  You might want to focus on sunsets or orange flowers, just to get that extra orange color into your room. Prints of Prairie lilies, falling leaves, or the beautiful orange glow of the sun across water would look great.  If you blow the prints up in black and white or sepia, take a felt and lightly color in a random flower or leaf orange.  It's a very beautiful effect.

Fill up the wall space by adding items such as a large clock, wall sconces that can be plugged in, or framed artwork. Once again, focus on deep greens or orange colors, so that your eye is drawn to the color on the wall and not the white.

Be sure to add orange pillows to the couch and curtains to the windows in your room. They will liven up the space with color and create an extra level of coziness. It sounds like it will be a great room, enjoy!