Staining that comes consistently within a day of the onset of flow, but does not make a woman niddah, has some halachic similarity to a veset. In such a case, veset calculations are still made based on the onset of the flow, but a woman may not have relations on the onah of staining. See here for more on veset and premenstrual staining.

Thus, you should count a day onah, but abstain from relations starting the previous night.

Please write back with any further questions.