I can’t find documentation on this, but it appears that you can edit this in the widget’s options panel, in the “Custom Event Format” box. It needs to be html of the form:
<li style="color:#333333; "><strong>%date-time%:</strong>%event-title%</li>
#333333 is the default color, so you can just replace that with whatever color code you want, and then paste the whole thing in the “Custom Event Format” box.