What Causes Recurring Pale Grey Patches On Thighs?

I am 75-year old male in generally good health. No known allergies. Subsequent to colostomy (Nov 14) and chemo-rad treatment for anal cancer (12 14 - 03- 15) have rec d all-clear from cancer clinic. After effect is most notably lymphedema in legs, presumably due to damage to lymph nodes in groin. Now experiencing pale gray-blue-ish stains in skin on thighs. No pain, no itching, no swelling, no heat. The colour appears as though dye from blue denim pants has bled into skin. However borders of the stained patches are well-defined, not blurry. These discoloured patches disappear and re-appear from time to time over a period of several days, not in same shape or location. They are in, not on, the skin. They cannot be washed or scrubbed off. Their shapes are irregular. Size ranges up to approx 1 x 3 . New GP (old one just retired) says she has never seen this. I have referral to dermatologist but likely a 2-month wait. Any suggestions?