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First off, Byungsoo, we commend you for venturing out and considering adding colour to your room walls. Often colour is the magic catalyst that takes a room from drab to fab. We've had to advise many friends and family about this very topic and we've got two recommendations.
For a quick, though inexact reference, you can "virtually" envision your room with the new wall colour by taking a digital photo and using an image editing application (Photoshop Elements or Microsoft Picture It are two options) to apply your selection of potential colours. This can be a first step reference to narrow down choices, since you can reference how the colours may or may not compliment your existing decor. Check out this Photoshop Elements tutorial for related information.
The next step we've used is through the aid of large stretched canvas, available at most every art supplies store. Purchase a canvas that will fill a great part of one of your walls in the shape and orientation and apply. By painting a canvas panel with your potential room colour and hanging it up onto the wall, you'll get a realistic idea of how the colour will look in different light and whether it really is a colour you can live with. It's sort of like going out on a date, but without the commitment to see this colour again if it doesn't work out. And remember, no wall colour is forever...if you really dislike it, you can always start with anew and repaint!