I just used the Paint like I do with all other works. And I do not like the scheme to use just "x" colors in my works. It's just a hobby...
It'd be nice to include the reference when you are posting something. Other than that, there's quite a lot of colors and they're not used efficiently. Out of 81 colors way over half are used less frequently than 0.5 %.. it could be how you're saving the image, or maybe you used some smudge tools in PS or something?
It's a hobby for quite a lot of us, me included. There are lots of different kinds of pixel art, but color conservation is still a key skill to have, it's not just an arbitrary limitation to make people's lives miserable. It'll make you think about color and palette more and how to use them to your benefit. For example, would you have thought about using light blue, bright yellow, purple and dark brown to shade her face? Those might not be colors you'd associate to a face normally, but DawnBringer's Marilyn uses them to great effect:
http://www.pixeljoint.com/pixelart/77867.htm
On the other hand, Katy Perry, even with all the colors, looks flat. It's missing darker shades to define the face. The pixelling is kinda cool but coloring is equally important.