Searching MySQL with PHP

[Crow]

Having two fields in a web page used for searching:

<tr><td align="right">
Description:</td><td><input type="text" name="description" value="" size="30" maxlength="30">
</td></tr><br>

<tr><td align="right">
Color:</td><td><input type="text" name="color" value="" size="30" maxlength="30">
</td></tr><br>

If I only fill in color and don't want it to search description what do I do? I have:
$sql="select id, description, color from tablename where description = '".$_REQUEST["description"]."' or color = '".$_REQUEST["color"]."'

If I just enter color blue and search I get this:
select id, description, color from tablename where description ='' or color='blue'
But I dont want to have description searcherd since i didn't enter anything.

Thoughts?