I think if you have a node tree like this:
Control
|-- HBuf (ColorRect)
|-- HCur (ColorRect)
Then you can use something like this:
if change <= 0:
$HBuf.show_behind_parent = true;
$HCur.show_behind_parent = false;
else:
$HBuf.show_behind_parent = false;
$HCur.show_behind_parent = true;
Then that should make whichever ColorRect node that has show_behind_parent set to true behind the ColorRect that does not have show_behind_parent set to true.
I have not tested the idea or the code, but I think it should work.