When you purchase hardwood flooring, you may additionally be interested in knowing if there is matching molding for the baseboard available, or if it's a bad idea to have baseboard that matches. In general, matching baseboard is not the best idea, but there are some instances in which you may find it to be an appealing option, so knowing what the visual effects of painted or stained baseboards are can help you decide what is best for you.
Matching Baseboard
If your baseboard matches the floor, then it will blend in well and seem like a coherent part of the floor. This may seem like a good thing, but it can create the unsettling illusion of seeming like your flooring is moving up or is part of the wall. However, at the same time, it has the more appealing illusion of making your floor seem bigger. You can get the effects of painted and matched baseboard at the same time by using painted baseboard with a quarter round trim at the base that matches the floor, though you may or may not like having two trims together that don't match.
Contrasting Baseboard
Baseboard that contrasts both with the floor color and the wall color (white is a common choice, but not the only effective choice) has the effect of defining the architecture of a room, drawing attention to structure and essentially outlining the room. It also clearly defines the division between the wall and the floor, which you may like. This has a positive visual effect and is a safe design choice if you're not sure what is best for your room.
Matching Walls
If you paint the baseboard to match the walls, then the baseboard will blend into the walls. This can have the effect of making walls seem larger, a positive quality for a small room. It also makes the architectural features blend together more, which is a good idea if you want to draw attention away from something. By reducing the number of colors to essentially the wall color and the floor, you also draw more attention to both, which highlights the floor and can be desirable if you have a particularly nice hardwood floor.
Matching Trim
If you have other natural wood trim in the room, such as window trim or chair molding, having the trim and the baseboards match the floor can lend a feeling of coherence throughout the room. The stained wood accents will tie in to one another throughout a room and complement one another. You may still wind up with the look of the floor going up the walls, but it will seem more like part of the overall design or decor of the room rather than an oddity.