The Decompose tool will decompose a colour image into a series of grey scale layers. This allows you to edit the image in many different colour models.
Go to Colours > Components > Decompose. This will bring up the Decompose Dialog.
The Decompose window looks like this:
Select the colour model that you wish to use from the drop down menu that matches the colour model you wish to split to.
When done editing your image, select Colours > Components > Recompose, to recompose the image back into an RGB Image. When saving to the XCF format, the layers will be preserved.