Use this custom CSS
.sp-megamenu-parent >li:nth-child(n) > a {
color: #8ac53f !important;
}
.sp-megamenu-parent >li:nth-child(2n) > a {
color: #2ab0ba !important;
}
.sp-megamenu-parent >li:nth-child(3n) > a {
color: #f1c712 !important;
}
.sp-megamenu-parent >li:nth-child(4n) > a {
color: #ed117d !important;
}
.sp-megamenu-parent >li:nth-child(5n) > a {
color: #92278f !important;
}
Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.