To edit the font color you need to change the CSS property that is link to your caption text.
To edit the CSS for the caption login to your wordpress site and depending on what version of wordpress your using go to >themes (or design tab) then theme editor > then click styelsheet.css
There might be a line similar to this one
.wp-caption p.wp-caption-text
{
font-size: 11px;
line-height: 17px;
padding: 0 4px 5px;
margin: 0;
color: #FFFFFF;
}
You would edit the color code where it says #FFFFF;
Hope that works!
Your Web Pros - Alex