Toolbar's background color? This can be done directly in AndroidStudio's view editor:
<net.dankito.richtexteditor.android.toolbar.GroupedCommandsEditorToolbar
android:id="@+id/topGroupedCommandsToolbar"
android:layout_width="match_parent"
android:layout_height="@dimen/editor_toolbar_default_height"
android:background="?attr/colorPrimary" // <- set your background color here
android:visibility="gone"
/>
Icon's color?
You can set this by toolbar's ToolbarCommandStyle.enabledTintColor :
editorToolbar.commandStyle.enabledTintColor = Color(<red>, <green>, <blue>) // red, green and blue in values from 0 to 255
What color do you want to change?