The site you pointed to seems to be mimicking deeper art forms like oil painting. Just like was already said, color isn't just one digital hue described by a hex-code. Oil painting have such a unique look because they colors are semi-transparent. It's the same thing with hair (why CGI hair is so hard to do), there's a transparency that is so much a part of the color. In PS you'll need layers, like everyone was saying. Play with the filters and textures to bring out the depth of the colors you're looking for.

I print my posters here with convenience