The menu icons are displayed via CSS in the :before pseudo-element of the .wp-menu-image.dashicons-before wrapper.
So first your code needs to target .wp-menu-image:before, or .dashicons-before:before (not .dashicons).
This won’t be enough though because default styles in WordPress are using some more specific selectors. Though you could override them with !important and make all icons red with this:
.wp-menu-image:before { color: red !important; }
(Note that this should be avoided whenever possible.)
But this is not what you are asking for here, you want to color only the icon for one specific custom post type.
You’ll need make your selector more specific to target your custom post type menu item.
If your custom post type slug is cpt_movie, this will do:
.menu-icon-cpt_movie .wp-menu-image:before { color: red; }
You may also want another color when you mouse-hover the menu item:
.menu-icon-cpt_movie:hover .wp-menu-image:before { color: green; }
Related quick tip: play around with your browser’s DevTools, it makes it really easy to work out which elements/classes you need to target in css 😉