Can't unselect taxonomy marker

Hi, I've set up taxonomy markers using a mid level taxonomy term. But I now want to use custom markers for the children of the taxonomy terms I'm currently using. However taxonomy markers appear to be set by the highest term in the vocabulary so just setting different colours for lower terms isn't sufficient, you need to unset the parent terms. Unfortunately this doesn't seem doable. If I select the blank setting in the taxonomy marker list for the parent terms and then save the term it just reverts back to whatever marker it was previously set to. I've tried clearing all caches and the marker cache but to no avail.

Also having difficulties getting the custom markers to appear in the list. But that's obviously something I've done wrong!