We appreciate the importance to you at this juncture of purchasing the garments that you prefer.
Even when a woman is
menopausal, uterine bleeding can make her
niddah. Wearing colored undergarments is a halachic measure which prevents stains from making a woman
niddah. (Please see our site's articles "
Stains" and "
Toilet Paper" for more details about staining.)
If you are prepared to take the risk of becoming
niddah should you have staining, then there is no reason not to wear white undergarments. If pastels might suit you, that could be a good compromise, since most halachic authorities consider pastel undergarments to be colored undergarments.
Please write back with any further questions.