Thanks for the clarification.
Juicebox uses the same custom characters for the main image navigation buttons and the thumbnail navigation buttons so it would not be possible to use a different character for main images vs thumbnails.
Also, the navigation button color options (navButtonIconColor, navButtonIconHoverColor, navButtonBackColor) affect both main image and thumbnail navigation buttons.
However, you could use CSS to change the color of only the thumbnail navigation buttons (for example).
If you wanted to use black arrows for the thumbnail navigation buttons, try something like the following in the <head> section of your gallery's HTML page.
<style type="text/css">
.jb-navigation.index-navigation .jbn-nav-button {
color: rgba(0,0,0,1) !important;
}
</style>