How can I preserve colour attributes when creating an NSAttributedString with RTF?

I am initialising an NSAttributedString with RTF and it works, in that I can see the text and attributes like font, size and weight are preserved.

NSMutableAttributedString* attributedRTFString = [NSMutableAttributedString.alloc initWithRTF:rtfContent documentAttributes:nil];

Some attributes, notably colour, are lost though. If I look at the RTF code I can see that those attributes are there (for example)…

{\*\htmltag72 </p>}
{\*\htmltag64 <p class=MsoNormal>}\htmlrtf {\htmlrtf0 
{\*\htmltag84 <b>}\htmlrtf {\b \htmlrtf0 
{\*\htmltag148 <span style='font-size:10.0pt;color:#5F5F5F'>}\htmlrtf {\htmlrtf0 
{\*\htmltag156 </span>}\htmlrtf }\htmlrtf0 
{\*\htmltag92 </b>}\htmlrtf }\htmlrtf0 
{\*\htmltag148 <span style='color:#1F497D'>}\htmlrtf {\htmlrtf0 
{\*\htmltag244 <o:p>}
{\*\htmltag252 </o:p>}
{\*\htmltag156 </span>}\htmlrtf }\htmlrtf0 \htmlrtf\par}\htmlrtf0
\htmlrtf \par
\htmlrtf0 

And I can see the colours in the original document.

How should I be opening RTF to preserve the formatting fully?