You can use contributed More font colours plugin to choose more font colors.
Set up this plugin for atto editor, go to Dashboard ► Site administration ► Plugins ► Text editors ► Atto HTML editor ► Atto toolbar settings ►Toolbar config ► write morefontcolors in accessibility i.e., accessibility = accessibilitychecker, accessibilityhelper, morefontcolors
Regards,
Purnendu