Can anxiety cause puple feet only when sitting or standing?

No. Anxiety by itself would not cause purple feet. Such discoloration may be related to venous stasis ( blood pooling in the lower limbs) and should be discussed with a primary care physician. It is understandable that a person might have anxiety with such a condition and that should also be discussed with the doctor.
Not usually. Anxiety does not cause purple feet per se. You may consider seing your doctor to make sure you do not have a circulatory problem or any other cinditions that could be causing your problem.