Title:
Rainbow Text

BB Code Tag Name
rainbow

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

Example:
text

Description:
Makes your text display in a rainbow gradient.

Options
Use {option}: No
Button Image:
Remove Tag if Empty: Yes
Disable BB Code Within This BB Code: Yes
Disable Smilies Within This BB Code: Yes
Disable Word Wrapping Within This BB Code: No
Disable Automatic Link Parsing Within This BB Code: Yes

Add this to your Custom CSS[SiteBuilder>Style>CSS
Or AdminCP>Styles>Style Manager>{style}>Edit Templates>CSS Templates>css_additional.css:
.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;
}

Thanks to noypiscripter for suggestion on custom css/span class

Would be better to stick the css in the css_additional.css template, so you don't have style tags getting inserted wherever the code is used.