<th id="sto">100</th>
<td onclick="ChangeColor( this, '#FEEC88' ); ChangeColor( document.getElementById("sto"), '#FEEC88' )"> </td>
без изменения JS. Либо так:
<th id="sto">100</th>
<td onclick="ChangeColor( [this, document.getElementById("sto")], '#FEEC88' )"> </td>
но тогда заменяем и JS:
var last_change = [];
var last_color = [];
function ChangeColor( elements, color ){
if (typeof element !== 'array') {
elements = [elements];
}
for (var i = elements.length; --i;) {
var element = elements[i];
if( last_change[element.id] != null ){
last_change[element.id].style.backgroundColor = last_color[element.id];
}
last_change[element.id] = element;
last_color[element.id] = element.style.backgroundColor;
element.style.backgroundColor = color;
}
}