Edit in JSFiddle

$("#color_p").change(function(){
    var value = this.value
    $("#color_c").children('option').each(function(i,n){
        if( value == $(n).attr('value') ){
            $(n).attr('disabled',true);
            $(n).css({'color':'graytext'});
        }else{
            $(n).attr('disabled',false);
            $(n).css({'color':'menutext'});
        }
    }).end().val('')
})
$("#color_c").change(function(){
    if( this.value != '' && this.options[this.selectedIndex].style.color == 'graytext'){
        this.value = '';
    }
});

<select title="" id="color_p" name="color_p" size="1">
<option selected="selected" value="">請選擇</option>
<option value="1">藍色</option>
<option value="2">紅色</option>
<option value="3">黃色</option>
</select>

<select title="" id="color_c" name="color_c" size="1">
<option selected="selected" value="">請選擇</option>
<option value="1">藍色</option>
<option value="2">紅色</option>
<option value="3">黃色</option>
</select>