Remove yellow stains on toilet
These appear to be rust stains.  I've had reasonable success with a product called Wink:
Wink on Amazon
there are also similar products that may be as effective but I have not tried them personally.  I do know that chlorine bleach does not work on these.
Be VERY careful with this product, however.  It contains HFl (hydrofluoric acid) which is nastier than HCl (hydrochloric acid).  Always follow the instructions carefully!