Can I customize my own map colors?

Yes. Click on Map, Map Styles. Select Fill Colors in the list on the left. This is where you can change colors.

Click Apply to see the changes immediately. When you are satisfied with the colors, you can choose Save As and assign your own style name.

TIP: In the Map Styles, there's a list of many available style color sets. Any styles that you may have previously saved will be listed here.