Stains that are light brown (the color of coffee with milk) with no reddish hue are acceptable. Other shades of brown should be brought for evaluation. If you have asked in the past about a specific shade of brown and were told it's acceptable, you do not have to ask again when you see another stain of that particular shade.
Whenever possible, stains should be evaluated in person and not via a photograph. In fact, most cameras do not capture color with anything close to the accuracy necessary for a halachic ruling. However, in situations like yours, where a woman does not have access to a halachic authority, we recommend the Tahor App. This app uses sophisticated color calibration technology to take a picture of a stain or bedikah and send it to a Rabbi for evaluation. Please note that, as of this writing, it is available for the iPhone and for a few models of Android phones.
If you are still unsure about the stained hefsek taharah and are unable to clarify its status, you should assume it was invalid and consider the next day's bedikah to be your new hefsek.
For the future, as long as it is not yet sundown, you may make additional attempts at a hefsek taharah until you have an acceptable hefsek. (This only applies to the hefsek taharah in order to start the seven clean days, but not to the bedikot during the clean days.)
Please feel free to get back to us with any further questions.