You can choose custom color from any Color dropdown menu in Sitebuilder. However, you'll need to know the RGB or Hex Color Code for the particular color to enter in the fields.
If you don't know the RGB or Hex information, you can use a tool like ColorCop to obtain the numbers.
Hope that helps!
Note: The link above is not from an official partner, nor have we done extensive testing on these products. We do not guarantee that instructions from this site will work for all users and cannot provide specific instructions or troubleshooting with these third-party websites. Use of these tools and services must be undertaken at your own risk.