I have several correspondents that use MS-Outlook mail clients, where some use Outlook 2003 and some use Outlook 2007. When I receive messages from people using Outlook 2003, the message looks fine (except that Outlook likes to use very small fonts for some weird reason), but when I receive messages from people using Outlook 2007, the fonts look fine but all color information is missing from the HTML rendering (I do get bold, underline and other text effects).

Please see attached screenshots for example. I can also provide the original emails personally, if needed.

From looking at the HTML message source, I can see that there are colors defined, for example - here's a dump from the emails shown in the screenshot:
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<p class=MsoNormal dir=LTR style='text-align:left;direction:ltr;unicode-bidi:
embed'><span style='color:#1F497D'><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal dir=LTR style='text-align:left;direction:ltr;unicode-bidi:
embed'><span style='color:#76923C'>Green</span><span style='color:#1F497D'>
&#8211; Passed. <o:p></o:p></span></p>

<p class=MsoNormal dir=LTR style='text-align:left;direction:ltr;unicode-bidi:
embed'><span style='color:red'>Red</span><span style='color:#1F497D'> &#8211;
Failed. <o:p></o:p></span></p>
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As can be seen in the screenshot, the red color for the word "Red" and the somewhat green color for the word "Green" are not rendered - instead the word appears as standard black.

Here is the same part but from the Outlook 2003 email. It seems that the main difference is that 2003 also ads "font" tags with their own color definition around the parts that should be colored.
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<p class=MsoNormal dir=LTR style='text-align:left;direction:ltr;unicode-bidi:
embed'><font size=2 color="#1f497d" face=Calibri><span style='font-size:11.0pt;
color:#1F497D'><o:p>&nbsp;</o:p></span></font></p>

<p class=MsoNormal dir=LTR style='text-align:left;direction:ltr;unicode-bidi:
embed'><font size=2 color="#76923c" face=Calibri><span style='font-size:11.0pt;
color:#76923C'>Green</span></font><font color="#1f497d"><span style='color:
#1F497D'> &#8211; Passed. <o:p></o:p></span></font></p>

<p class=MsoNormal dir=LTR style='text-align:left;direction:ltr;unicode-bidi:
embed'><font size=2 color=red face=Calibri><span style='font-size:11.0pt;
color:red'>Red</span></font><font color="#1f497d"><span style='color:#1F497D'> &#8211;
Failed. <o:p></o:p></span></font></p>
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