I think they are allowed. That is of course if you are talking about a solid colored cube with stickers, not a cube with colored plastic replacing stickers.
Here are the official rules for puzzles: http://worldcubeassociation.org/regulations/#puzzles
3a) Competitors must bring and use their own puzzles. Competitors may borrow puzzles from other competitors, although both competitors must be ready to compete when they are called for to do so.
3b) Puzzles must be in reasonable working order, so that normal scrambling is possible.
3c) Competitors must use any colour scheme for cube puzzles, as long as the puzzles show one colour per face in solved state. For other puzzles competitors must use any variation that has the same moves, positions and solutions as the original puzzle.
3d) Puzzles must either have coloured stickers, coloured tiles, textures or painted colours.
3d1) Puzzles using textures must have different colours on each face to aid in scrambling and judging.
3e) The colours of puzzles must be solid, the same per colour, and clearly distinct from other colours.
3e1) The stickers/tiles/textures/paint may show an image (in one colour), as long as all stickers/tiles/textures/paint of a colour have the image and the same image.
3f) Stickers/tiles/textures/paint must not be thicker than 1.5 mm, or the generally available thickness for non cube puzzles.
3g) Puzzles may be made smoother internally by sanding or using any lubricant.
3h) No modifications are allowed that enhance the basic concept of a puzzle. Some examples of enhancing the basic concept are: new moves are possible, normal moves are impossible, more pieces are visible, colours on the backside of the puzzle are visible, moves are done automatically, more or other solved states.
3i) Any modifications to a puzzle that result in poor performance or results by a competitor are not grounds for additional attempts in an event.
3j) Puzzles must be clean and must not have any markings, elevated pieces, damages, or other differences that distinguish one piece from a similar piece.
3k) Puzzles must be approved by a judge before use in the competition.
3l) Cube puzzles must have at most one logo. For Rubik's Cube or bigger cube puzzles the logo must be placed on one of the centre pieces.
3l1) Colourless logos (max. 1 logo per colour) that are engraved are not considered as logos.
3m) All puzzle and puzzle part brands are allowed, as long as the puzzles comply with the other WCA regulations.