Created attachment 60854 [details]
sample file

In the attached file in cell A1 we have the string "abc":

in content.xml
<table:table-cell table:style-name="Gnumeric-default" office:value-type="string">
<text:p>a<text:span text:style-name="NS-colour-ff0000">b</text:span>c</text:p>
</table:table-cell>

The style-name "NS-colour-ff0000" refers to the named style given in styles.xml:

<style:style style:name="NS-colour-ff0000" style:family="text" style:display-name="Font Color #ff0000">
      <style:text-properties fo:color="#ff0000"/>
</style:style>

This should cause the "b" in A1 to be red, but it shows as regular black.