It depends if you want to change the colour of the header row on all tables, or on a table-by-table basis.
If the former, you can use the colour palette editor in Control Panel to redefine "table-header-background".
If the latter, the easiest approach is to use CSS. First, add a CSS class to your table with a special "k" row:
|myTableClass|k
|!One |!Two |
|Three |Four |
Then define a stylesheet by creating a new tiddler and tagging it "$:/tags/Stylesheet", with the following content:
.myTableClass th {
background: red;
}
If you wanted to change the colour of ordinary table cells the rule would be ".myTableClass td".
Best wishes
Jeremy.