Rugs and carpet are a big investment and will be part of your interior decoration for a long time. In my opinion, flooring should not be a fashion statement, it should be timeless. As a result, I prefer simple style rugs in indoor living & bedrooms areas, to provide a neutral contrast to soft furnishings whilst creating a serene & relaxed atmosphere. Also living areas need to be considered as high traffic areas with children and pets and so the style of fibre rug will also come in to play.
I recommend incorporating colour and pattern on the floor in hallways, entrances, play rooms, bathrooms or staircases as they hold their own in these type of spaces and you're not working around them to decorate.
Rugs on wooden flooring or marble provide a more contemporary look vs wall to wall carpet. Don't be afraid to mix textures on a rug - put a leather border with a natural fibre for example.
In a bedroom if the tone is grey, then I would recommend a rug in the same colour to provide consistency with the whole room. It is important to make sure that the rug covers half of the bed and comes out far enough each side that you step on to the rug when you are getting out of it. The rug should also come past the end of the bed.
Adding a rug in to a kitchen adds variation to it and makes it less sterile. A patterned rug also makes a kitchen fluid and stylish. In the open plan design of most apartments/homes, it means you no longer need that tradition of placing a rug under the dining table. This makes smaller open living/dining areas seem larger. I prefer to centre a dining table in open plan living with a hanging pendant light.
The size of the rug plays a large part in grounding the room. If you purchase a rug that is too small then it makes the overall room look small. In a living room, the rug defines the conversation area and it should be large enough so all the chairs/ottomans are included so that at least the front legs of the furniture are on it. If you are in a rental a good way to trick the size is to buy 2 smaller rugs and place them together to create a larger mass area.
Layering rugs is a short term fix for those that have a low boredom threshold (like myself). I love using cowhide rugs or striped rugs over natural flooring to add some texture and pattern (shape of the rug).
Finally I always recommend placing an underlay under the rugs to provide a more plush feel, it also helps anchor the rug to prevent it from slipping and keeps the rug cleaner for longer in the tropics. The type of underlay you use is important so that it does not absorb water from the humid air. Absorption of water means the dust and dirt will stain the rug requiring more professional cleaning maintenance.