See this post: RT eats table-tags in html-emails - #8 by Guadagnino_Cristiano
By default RT scrubs HTML attributes that are not explicitly aloud for security reasons, documenation is here: RT::Interface::Web - RT 4.4.4 Documentation - Best Practical
In your Web_Local.pm callback I think you can get started with this as the content:
package HTML::Mason::Commands;
# Let tables through
push @SCRUBBER_ALLOWED_TAGS, qw(TABLE THEAD TBODY TFOOT TR TD TH);
$SCRUBBER_ALLOWED_ATTRIBUTES{bgcolor} = '^\s*#[a-fA-F0-9]{3,6}\s*';
1;