After sclerotherapy treatments, when does the brownish color go away? (photo)

The dark staining after sclerotherapy is not uncommon.  it can last for months, or longer, but it usually does lighten significantly over time.  I would keep the appointment and show your treating clinician the spots you are concerned about and see what he/she has to say.  You can always opt to not have a treatment after being examined.  However, if there are other veins that remain, usually having them treated will improve the overall appearance while you are waiting for the staining to fade.

Hyperpigmentation of the skin can occur after sclerotherapy in anyone and is a known side effect of traditional or foam sclerotherapy. It is more likely when reticular veins are injected and also when large veins under the skin (varicose veins > 6 mm) are injected. Proper compression with stockings and strict avoidance of sun exposure are necessary components of therapy as well as the use of topical products. I recommend Scleroquin plus and Sclerovase creams.  see the link below. 

What your pictures show is called staining which is a result of what is called hemosiderin release from trapped or extravasated blood.  This can occur following sclerotherapy and, usually resolves spontaneously. It hopefully will resolve sooner but it could take up to a year and, very occasionally, it will be permanent.  There are several things that can be done to help to resolve this-first your doctor should see if there is any area in which he can release the trapped blood. Second-your doctor can look for any open feeding veins and close them.  If no improvement from this then a venous reflux exam should be done to ensure that that are no refluxing valves which may be contributing to the back pressure. Although, in general, bleaching creams usually don't work, trying hydroquinolone may help. I have also used in the past (at the advice of a midwest farmer), an over the counter item call bag balm which also has hydroquinolone, is inexpensive and easy to apply.  You have nothing to lose by trying this for 2 to 3 months.