If the retzuous (straps) of your tefillin are not kosher, you are not fullfilling the mitzvah of tefillin. These pictures show examples of posel retzuous.
I do not think these retzuous are kosher as there is a large brown spot on them. After painting they will be fit for use.
It took me a while to become convinced of the value of retzuous that were black on both sides. At first I thought it was a gimick. After a while