I believe you can't have custom color highlights (you can change color only for all people of a gender) but you can have a "direct ancestor-only" view.
For this, you need to click on the "Options" tab (bottom of your screen). On the second column from the right, you can customize your tree view - how many generations of ancestors, how many generations of descendants, and "other". "Other" basically means "every family member who is neither a direct descendant or ancestor", i. e. nephews, nieces, cousins, aunts and uncles. If you select "none", you'll only see your ancestors (possibly descendants) and siblings (if you have some).
You always reverse this setting, and it only changes visibility, not how many people are on the tree.
If you want to highlight a specific line of descendance (say, you want to highlight how exactly you are related to your great-grandfather), you can click on the family member in question and click on "Show relation to [family member's name]" - this would highlight the trail of people through whom you are related to that family member.
Hope this helps!
