A Dallas home as design lab

An interior designer's home is not only his castle. It's also his laboratory, where he experiments and refines ideas and techniques. Or so it is with Gerald Tomlin, a Dallas designer who keeps developing his own home to suit himself, his wife Joanne, and their three sons as all their interests change and expand.

Since they all enjoy living with a fairly neutral color scheme, it is imperative, the designer contends, to have contrasts of textures on walls, floors, ceilings, and furniture.

''I find that the quality of slickness on every single surface is boring, so I introduce many textures in a room, even to combining several different levels and weaves in a single rug. I contrast velvets with suede leathers, Indian silks , smooth vinyls, and deep-piled fur throws. In my living room I have a floating ceiling of bronze mirror, which reflects the patterned Mahal area rug, the 18 th-century bronzes, and the 17th-century Chinese paintings on rice paper.''

What gives him enormous satisfaction, he says, are all the interesting textural effects he can get with the use of ''simple, mundane materials.''

Take plaster, for instance. He buys premixed bedding compound (a plaster-like substance that comes in a big can) from the paint store. Using a slightly dampened shaggy roller, he rolls the bedding compound onto a wall with criss-cross motions.

While the plaster is still wet, Mr. Tomlin trowels it, creating a textural surface. After the wall dries thoroughly, he gives it a coat of glossy oil-based enamel in a color he chooses. The enamel not only seals the plaster-like compound but also becomes the base for the coat of glaze that is then wiped on with cheesecloth and worked into the texture. A final coat of varnish, he says, produces a rock-hard wall that will last for years and is totally washable.

The somewhat tired-looking cherry wood paneling of the Tomlins' study is now a putty white with a striated dimensional texture similar to barn siding. The designer gained this effect by dry brushing gesso, straight out of the can, onto the paneled walls and allowing it to dry.

To get different moods in a room, Mr. Tomlin experiments with inexpensive wall washers that will cast interesting patterns of light. ''I buy plain radiator-cover metal from the hardware store, and with metal shears cut out circles and slip them in front of the light source of any track lighting system, '' he says, ''which enables you to direct light directly onto a wall. Sometimes I use two circles, in order to overlay one pattern over another and get an effect of lacy leaves.'' He calls the technique lightscaping and finds it an exciting tool for changing the ambiance of a room.

Mr. Tomlin defines and focuses the conversational area of his own living room with a floating ceiling, which is a rectangular-shaped frame of bronze mirror. His dining-room ceiling appears to be a sea of tiny reflective copper mirrors. His master bedroom ceiling is covered with cloudlike silver Chinese tea chest paper, lightly and ethereally dusted with silver and gold metallic dust.

In another bedroom, he gets the rolling cloud effects of an old Italian fresco by using an air-pressure gun to ''fog on'' glazing liquid around the perimeter of the room. In garden-like settings, he is also fond of using latticework ceilings.

As for teen-age boys' rooms, he recognizes their love for computers, electronics, posters, waterbeds, and multifunctional storage spaces. Consequently, son Gerald's room is a marvel of engineering that features a central control system at his fingertips by which he can open up the draperies, turn on the ceiling fan, and control the temperature, the television and the stereo, the lighting system, and the computer.

Walls are paneled with Celotex, then covered with glued-on blue twill. This converts all walls into tackboards that can handle, without damage, any number of snapshots, posters, and pennants. A second teen-age son's all-gray room has walls covered with gray flannel suiting, a waterbed set into a lighted platform, a projection instrument that revolves and throws light all around the room, and a chrome and black leather classic Eames chair.

Mr. Tomlin, a graduate of the University of Rochester and Parsons School of Design in New York and a member of the American Society of Interior Designers, has his own interior design firm in Dallas.