@Tropoje $replace is case sensitive, but you can do it over infinite number of arguments, so just make replacements for both cases.
$if($or($strstr(%title%,feat.),$strstr(%title%,Feat.)),$replace(%title%,feat.,$colour{128-255-0,128-255-0}feat.$colour{},Feat.,$colour{128-255-0,128-255-0}Feat.$colour{}),%title%[ '('$colour{128-255-0,128-255-0}feat.$colour{} %featuring%')'])
Thank you very much for your quick reply
My goal is to always make changes Feat. (uppercase) to feat. (lowercase). I have slightly modified your code and now it works as it should. This solves my problem
$if($or($strstr(%title%,feat.),$strstr(%title%,Feat.)),$replace(%title%,Feat.,$colour{128-255-0,128-255-0}feat.$colour{},feat.,$colour{128-255-0,128-255-0}feat.$colour{}),%title%[ '('$colour{128-255-0,128-255-0}feat.$colour{} %featuring%')'])