For me, when choosing a rug, colour is the obvious starting point. You may already have a lot of colour or pattern in your space, so you might want to stick with a monotone or single coloured rug. Monotones or neutral shades will complement your existing furnishings. In short, the rug becomes a subtle layer.
If you are more confident and want to add patterned or bold colour, it is easier to choose the rug first and then build up the overall design of the space selecting furniture and accessories to compliment the bold design.
A rug will always read as one of the largest ‘objects’ in a room so be sure to take into account the colour of existing flooring, walls and other fixtures that cannot be changed. But don’t play it too safe: a boldly patterned or brightly coloured rug can transform a room.