Generally addressing the color of the default Collapse/Expand text:
.mw-collapsible-toggle-default a { color:black; }
You may want to add the page's class if it's supposed to be applied on one or certain pages only. The code above would apply to all collapsible toggles everywhere on the wiki.
Perhaps you would also want to apply it only to the light or dark theme, or differently depending on the theme, by using
.theme-fandomdesktop-light
.theme-fandomdesktop-dark
Finally, you may also want to change the color of the brackets the text is wrapped in with something like
.mw-collapsible-toggle-default::before,
.mw-collapsible-toggle-default::after {
color: black;
}
A link to your page would as always be helpful to give you a more in-depth answer.
See also: https://community.fandom.com/wiki/Help:Collapsing