The hefsek taharah indeed does not need to be completely clear to be considered acceptable; however, orangey colors are generally problematic. While the lighting in the bathroom may have affected your perception of the color of the stain, since you no longer have the cloth to be evaluated, and this is an internal bedikah (which is treated more strictly than a stain found externally), you should assume that it was not valid and you need to perform a new hefsek taharah. If you got an acceptable bedikah after this attempted hefsek taharah, it can be counted as your new hefsek taharah.

For the future, when faced with a questionable stain, you should bring it to a rabbi for evaluation. Even if the stain is on an undershirt you should bring it it, or if you are uncomfortable, you can cut out the stained part of the undershirt to bring for evaluation, noting it was used for a bedikah.

Please feel free to get back to us with any further questions.