Keep in mind using styling like that in emails is a bad idea.
Providers like gmail, yahoo, msn and web-based clients like roundcube and that other one will likely strip it out. Instead, use inline styles.
<h1 style="color: baby-snot-green;size: huge;">Stuff</h1>
Edit:
Heres some code that works for emails... No <p> styling though... Do that in spans. Email = very very finnicky.
Code:
<P>
<SPAN style="LINE-HEIGHT: 110%; FONT-FAMILY: arial; COLOR: #3ddd5b; FONT-SIZE: 22px; FONT-WEIGHT: bold">
Some header text
</SPAN><BR>
And some long paragraph text.
</P>