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.
Retzuous That Have Faded And Appear Brown
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.
Ultra Double Black Retzuous
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