I want to put a span in the title so I can change the color of that span by CSS, but by default, every html tags in k2 item title will be stripped after the item is saved, how do I prevent this?

Hello,

Unfortunately HTML in titles is not supported. A 'hack' would be insterting a symbol and using PHP's explode www.w3schools.com/php/func_string_explode.asp to seperate the title.