First of all, I'd suggest not to put HTML into your datasource. I am sure you can find a better solution to submit additional information (like the color).
You can get the desired result if you make an additional call from your template and parse the string from the datasource:
<script id="groupHeaderTemplate" type="text/x-kendo-template">
<strong style="color: #=color#">#=doMagic(text)#</strong>
</script>
<script>
function doMagic(s) {
return (new DOMParser()).parseFromString(s, 'text/html').body.textContent;
}
</script>
(Kudos to Decode & back to & in JavaScript)