Designations in docbook for conversion to ePub

I have a well formed book in docbook XML format. I was wondering how to solve two issues:

First, what designation can I use for color attributes for text? I would like it to convert over to EPub3 with a CSS that has something like span.red {font-color: red;}

Second, I have several images with specified widths in the XML file, coded like:

 <para><inlinemediaobject>
    <imageobject>
     <imagedata fileref=“figures/TestImage.jpg" width="50%"/>
    </imageobject>
   </inlinemediaobject></para>

This is fine in docbook but when I convert this to ePub, it generates ePub errors - as XHTML cannot specify image width directly. To be error free, I need to specify with width  through a CSS designation like img.half where img.half is set to 50% width in the CSS. Is there a way in Oxygen XML to specify a designation in docbook that will at least tag these images so that I can add the CSS specification for width?