Hi,
Well, the cause of the yellowish or brown 
vaginal discharge is a common and usually harmless issue. Occasional 
spotting between periods might have this appearance and can be normal, especially if you're taking low-dose birth control pills. 
So in most cases this is not sign of disease. If it is, then usually this indicates 
bacterial vaginosis, STD or 
polycystic ovarian syndrome. 
Less common, some other 
hormonal disorders may be present.  In some cases, however, a yellow or brown vaginal discharge could indicate a serious problem. 
Although brown and yellow vaginal discharge often does not indicate a serious problem, it should not be ignored if it persists, recurs or is accompanied by other symptoms and you should see a gynecologist for further work up.
In some cases, a yellow or brown vaginal discharge could indicate a serious problem.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr Ivan R. Rommstein, General Surgeon