Your question raises an important point and is a large part of the reason for the establishment of the yoatzot halacha. However, we want to differentiate between different levels of training.
On a personal level, a woman can gain a limited amount of experience in evaluating her own stains. Colors that do not present a question – clear, white, yellow, or light brown discharges (the color of coffee with milk, with no reddish hue) – need not be shown to a rabbi. A bedikah that has a bright red stain doesn't usually have to be looked at either, because it is obviously a niddah color (except if it comes at an unexpected time or under special circumstances, such as after a medical procedure, which may not make you niddah). Any color which is unclear – such as darker or reddish browns, or pink – should be evaluated by a halachic expert in this field. If the same 'unclear' color recurs, you may ask your rabbi if this is a color which you have to bring to him in the future or if you can extrapolate from his recent ruling. In this way, your rabbi can train you over time where possible.
For the 'unclear colors' described above, extensive training involving practical experience (known as shimush) is needed. This training includes learning from a rabbi which colors are acceptable in the tradition he has received. This requires a significant dedication of time — after having learned the halachic principles. Some yoatzot halacha have already completed this additional level of training, and additional ones may do so in the future. All yoatzot are available to help facilitate the asking of the question so that a woman who is uncomfortable showing her stains directly to a rabbi does not have to do so. Over time, with more and more women dedicating time to Torah learning, there will likely be more and more women available to provide this service. Furthermore, all women should spend time learning the basic halachot to help minimize the times that they need to ask a question. One way to start is with our Marriage Companion online course.
Please feel free to get back to us with any further questions.