function wstawtag(txtarea, tagOpen, tagClose, sampleText)
{
    // IE
    if(document.selection  && !is_gecko)
    {
        var theSelection = document.selection.createRange().text;
        if(!theSelection)
        {
            theSelection=sampleText;
        }
        txtarea.focus();
        if(theSelection.charAt(theSelection.length - 1) == \" \")
        {
            // exclude ending space char, if any
            theSelection = theSelection.substring(0, theSelection.length - 1);
            document.selection.createRange().text = tagOpen + theSelection + tagClose + \" \";
        }
        else
        {
            document.selection.createRange().text = tagOpen + theSelection + tagClose;
        }

        // Mozilla
    }
    else if(txtarea.selectionStart || txtarea.selectionStart == '0')
    {
        var replaced = false;
        var startPos = txtarea.selectionStart;
        var endPos = txtarea.selectionEnd;
        if(endPos-startPos) replaced=true;
        var scrollTop=txtarea.scrollTop;
        var myText = (txtarea.value).substring(startPos, endPos);
        if(!myText)
        {
            myText=sampleText;
        }
        if(myText.charAt(myText.length - 1) == \" \")
        {
            // exclude ending space char, if any
            subst = tagOpen + myText.substring(0, (myText.length - 1)) + tagClose + \" \";
        }
        else
        {
            subst = tagOpen + myText + tagClose;
        }
        txtarea.value = txtarea.value.substring(0, startPos) + subst +
          txtarea.value.substring(endPos, txtarea.value.length);
        txtarea.focus();
        //set new selection
        if(replaced)
        {
            var cPos=startPos+(tagOpen.length+myText.length+tagClose.length);
            txtarea.selectionStart=cPos;
            txtarea.selectionEnd=cPos;
        }
        else
        {
            txtarea.selectionStart=startPos+tagOpen.length;
            txtarea.selectionEnd=startPos+tagOpen.length+myText.length;
        }
        txtarea.scrollTop=scrollTop;
    }
    // reposition cursor if possible
    if (txtarea.createTextRange) txtarea.caretPos = document.selection.createRange().duplicate();
}

<select size=\"1\" name=\"kolor\">
      <option value=\"kasztanowy\" style=\"background-color: #800000\" onclick=\"wstawtag(x.tresc, \"[kolor]\",\"[/kolor]\",\"\");\">Kasztanowy</option>
...

<input type=\"button\" value=\"[ b ]\" onclick=\"wstawtag(x.tresc, \"[b]\",\"[/b]\",\"\");\">