Hi Miho, love the study, great brushwork and I also really like the focus on eye on our right, with the other eye being softer, very classic. In terms of your question "how to make the colors less boring" I think you're already on the right track. I see the colour notes of purples, blues, and red in your shadows you could maybe emphasize this more? Or give colournotes to the light areas - perhaps following some general guidelines for colour zones of the face (top of face generally more yellow, middle generally more red - especially ears, bottom of face generally cooler colour temperature). You could also add some colour variety in the background. These are all just suggestions, I think it's perfectly well for a portrait study. One other critique, although I really love the loose brushwork, especially as it goes away from the focal point it could be mistaken for hair near the clavicles - just something I noticed! The brushwork is very uniformly small shapes down there could be why I read it that way.

This is a great start! The structure is feeling good and the markmaking is really lovely. As far as making the colors a bit more exciting; I would focus mostly on creating temperature contrast. If you darken the skin just a touch you'll have more room to get some lovely cool highlights in there, influenced from the background. Then I would keep an eye on any downward facing planes / shadow edges for opportunities for reflected light and subsurface scattering to create warmer tones for contrast. You can also push the cool tones even further if you want a more stylized finish; take a look at Steven Assael's work for some great examples. Also I'd keep an eye on so heavily separating the head from the neck; it's hard to say for sure without seeing the reference, but it looks like you have the jaw rolling into full shadow but its value is much lighter than the other shadows. I would probably darken it down and then bring the chin back out with reflected light. Great piece, keep it up!! :D