It depends on what is the type of $aVal. If you are expecting it to be a string then you can write:
function FCallback($aVal) {
if (strcmp($aVal, '0') == 0) $c = "red";
elseif (strcmp($aVal, '0.00')) $c = "black";
else $c = "green";
return array(3, "", $c);
}
If you are expecting the type to be a numeric type, then you can use operator=== to compare.