Improve color view

[akrah]

Thanks a lot for contributing to DGtalTools-contrib, before submitting your PR, please fill up the description and make sure that all checkboxes are checked.

PR Description

• Default color to white
• When you use "-c" option, tubes are colored as balls

  Checklist

• [ ] Doxygen documentation of the code completed (classes, methods, types, members...)
• [ ] Check if it follows the tools structure described in CONTRIBUTING.md
• [ ] New entry in the ChangeLog.md added.
• [ ] Update the readme with potentially a screenshot of the tools if it applies.
• [ ] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).

[kerautret]

looks fine thanks!