For what it may be worth, I see the same thing. Rebeccapurple does not work but #663399 does work. This has been reported to Kobo.
Code:
<td style="background-color:rebeccapurple;color:white;" title="rebeccapurple = #663399">
rebeccapurple
</td>
</tr>
Code:
<td style="background-color:#663399;color:white;" title="rebeccapurple = #663399">
rebeccapurple
</td>
</tr>