Thank you for reaching out to us.
1) If the stain on your underwear disrupted your clean days, then the bedikah that you performed on Tuesday morning can count as a hefsek taharah, with immersion rescheduled for Tuesday night.
If Covid is preventing you from asking a she’elah, then we suggest that you call your local halachic authority to see if you can clarify your status verbally, by describing the shade. Alternatively, you can have the stain evaluated online through Tahorapp (available on iPhones and on some Android phones), which uses special technology to ensure accurate transmission of colors to a halachic authority. A shade of brown lighter than coffee with milk, with no hint of red, is an acceptable color and does not require evaluation.
2) Generally speaking, one should not opt for stringency when it is possible to ask a she’elah. This type of stringency is often a leniency in shalom bayit. A couple of days here or there add up over time.
3) When a stain on an undergarment is clearly less than a gris in size, then it may be disregarded, and you need not ask about it. On bedikah cloths, the size does not make a difference, so you should ask about a questionable color regardless of its size.
Please let us know if we can be of further assistance.