I am no expert and I openly admit this, but I can easily see that her posture is a bit too rigid. From what I've learned to date about pose drawing, is that it needs to look entirely natural. I understand the idea of what "royalty" goes through to appear perfect or flawless, but her left arm only looks like it's dislocated. The line work on everything else is great and your color scheme works like it is supposed to. I can easily see the color harmony you were going for and it's done well. However, the posture of your character detracts from the overall composition. I do also notice that some of the shading seems to make it look there is more than a single light source.
For your first time using Touch Twin Markers, this is a masterpiece! ^-^ I love the vibrant colors you chose, and the overall design of the picture is really good! The shading is really nice, and usually if you're using a particular type of markers for the first time, it would be really hard to control the ink from spreading everywhere, but you managed to make the colors blend well nicely! However, I feel that the posture of the girl is a bit unrealistic, it would be kind of uncomfortable for a real person XD. Otherwise, the whole picture is gorgeous!