The laws of taharat hamishpacha require white underpants during shivah neki'im, both so that any staining will be clearly evident and in order to reflect a woman's awareness that she is in the midst of her shivah neki'im.
After the fact, so long as there was continuing awareness of counting the clean days, forgetting initially to switch to whites does not disqualify them. So long as you continued to perform bedikot, you established that there was no break in your awareness. Your hefsek remains valid and you may continue counting.