Sometimes muted, subdued colors can be fun. Ever try out the Prismacolor gray range? You can get some slightly warm and slightly cool tones without veering too far from the neutral. It can also be fun to stick almost entirely with neutrals but then introduce one vibrant color element, such as primary red.
The skull is good, nice perspective. Doing a quick review of your older work, I'd say that anatomy is the most conspicuous thing- for example, a couple pieces featured eyes prominently- if the eye anatomy in those pieces had been as accurate as the skull is here, they would have been stronger pieces in general. In other words, thumbs-up in the attention to anatomical detail.
Another comment, as long as I'm dishing out the unsolicited advice. Some of your strongest work uses the full value range effectively, but not all your pieces do. In this one you've got your dark detailing and some very soft shading, but the mid-range values are underrepresented. Sometimes it's nice to sketch out your initial detailing in mid grays instead of blacks, then to build the whole thing up until you arrive at your darkest tones in just the right places. That makes for a much fuller, deeper look. Also try looking at the piece from a distance every now and then as you are building the shading, which gives a better sense of how the depth and volume reads.