Paint plain plastic covers to exactly match your tile design for a seamless look. Lightly sand the cover and prime it with a plastic primer to ensure a good-adhering surface. Have the paint color-matched in the same sheen as the tiles, and seal with a polyurethane or other sealer.
Use metal spray paint to dress up plain plastic covers in a more contemporary setting. For example, design your outlets to look like chrome against a glass tiled backsplash or gray stone against a flat black industrially styled one. There are many faux finishing spray paints you can use to dress up your outlet covers with a durable finish.
Cover your outlets in fabric in a complementary pattern that adds another dimension to the overall look. For example, if you have a black and white tiled backsplash, consider a modern zebra-striped pattern or a retro black and white flowered pattern to match the style of your room.
Design a custom outlet cover with scraps of vintage wallpaper, magazine ads or other paper memorabilia. Use a decoupage medium, found at any craft store, to adhere and seal your design. Bring in a theme with items such as fruit or wine labels or an accent color such as a red scroll pattern on an otherwise completely white tiled backsplash.
Add removable wall decals to your outlet covers if you cannot decide on a permanent design. Cut the decals to the size of your covers, with a little extra over the edges that you can tuck under before putting the covers back on the wall. When you want to change the design, you just take off the cover, peel off the decal and replace it with another one.