Remember when it comes to the TCG GB that your palette is very limited;
and by limited, I mean like four colors max (just the card's artwork itself, that is). You have used 18 here. Now, I'm just going to be referring to the palette limitation for the first Pokemon TCG Gameboy game at the moment.
If you'd like to conserve your colors to four colors, you're likely just going to have to stick with the basics: a very light color, a very dark color, and two regular / base colors. In this case we would probably use a pale, light yellow for the "white" slot (the official sprites never appear to use pure black or pure white in their sprites); a darker, reddish-orange for the "black" slot, and two oranges. It would likely end up looking something like this:
Forgive me for doing a terrible job with that, but hopefully I am putting my point across successfully!
Now, to go over the palette restrictions for its sequel: Pokemon Trading Card Game 2 for Gameboy Color (if I'm not mistaken). Due to the gameboy colors slightly enhanced graphics,
more colors are permitted for each piece; about 8. While it's only a +4 increment, it makes a gigantic difference! Let me show you with the same example:
In this you can probably see that I got rid of several redundant colors (ones that were only used once or twice that could easily be replaced by an already existing, more widely used one), and altered others. I got rid of the pink on the tongue and just replaced it with the orange and the dark reddish-orange; it still has a similar effect, if not the exact same effect as it did when it had a pink tongue. By getting rid of this pink color, I had just enough slots to be able to keep the two greens in the background. The blue in its eyes and wing membranes were left as is.
So in conclusion, this
doesn't look much different than this
AND the second one uses fewer colors! Awesome. Sometimes things don't run so smoothly, though!
What you always want to keep in mind that if you're restricted on your color amount, you've gotta rule out redundant colors. It's really all just trial and error! Keep messing around with the colors until you find the colors that look the best. When you have a limited number of colors to work with, it's all about functionality!
I apologize if I made an unnecessarily long post ranting on about the guidelines on a style that you probably aren't even trying to follow so strictly. Nonetheless, the things that have been mentioned in this post can be applied to so many things!
I'd love to see more of these from you, please do keep us updated on them. :>