Changed to classes
I'm now using this:
code:
<script>
   function hide(id){
      document.getElementById(id).style.display = 'none';
   }    
   function show(id){
      document.getElementById(id).style.display = '';
   }
   function spoilerhide(){
      for (i=0;i<document.getElementsByTagName('div').length;i++) {
         if (document.getElementsByTagName('div')[i].class == 'spoiler') {
            document.getElementsByTagName('div')[i].style.color = '#000000';
         }
      }
   }
   function spoilershow(){
      for (i=0;i<document.getElementsByTagName('div').length;i++) {
         if (document.getElementsByTagName('div')[i].class == 'spoiler') {
            document.getElementsByTagName('div')[i].style.color = '#FF0000';
         }
      }
   }
</script>
<div id="div1">
    <input type="button" onClick="hide('div1'); show('div2'); spoilershow();" value="Laat spoilers zien">
</div>
<div id="div2" style="display:none;">
    <input type="button" onClick="show('div1'); hide('div2'); spoilerhide();" value="Verberg spoilers">
</div>
But it still doesn't work, and with
code:
if (document.getElementsByTagName('div')[i].class = 'spoiler')
instead of
code:
if (document.getElementsByTagName('div')[i].class == 'spoiler')
It makes all div's white and black 
