Since the colors of the caps on most markers look nothing like the actual color of the markers, I usually make my own color chart.
Nothing fancy, I just type the color name/number in a couple of colums and print it out on cardstock. Then I make a little square of the color next to it. I have a set of Bics and Sharpies and I made one for them, too.
Then I can look at the chart and see what the color actually looks like.
I also usually stamp a couple of extra images on scrap cardstock when I'm actually making a card. So I can "practice" on the image and see what my color choices actually look like together.
Finally, don't be too hard on yourself. This hobby is for fun! Most of the people I send my cards to are not "artists" and do not look at how the cards are colored with nearly the critical eyes that we do ourselves!
I have a good friend that I send cards to quite a bit. We were having lunch one day, and she happened to have a card in her purse that she had just gotten from the mailbox on the way to lunch. She was thanking me, and I said something about the colors. She looked "astonished" and said, "Oh my gosh - You color these? I thought they came that way when you stamped them."