[noypiscripter]

I suggest you use this replacement in the custom BBCode. Do not use custom tag <rainbow> as IE (especially older versions) may or may not like it.

<span class="rainbow">{param}</span>

Then add the below custom CSS in css_additional template in AdminCP or via Sitebuilder > Style > CSS Editor:

.rainbow {
    background-image: -webkit-gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) );
    background-image: gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) );
    color:transparent;
    -webkit-background-clip: text;
    background-clip: text;
}