Can interstitial cystitis cause dark yellow urine?

Not directly. Symptoms of interstitial cystits include lower abdominal pain relieved with urination, an urge to urinate and frequent urination. Dark yellow urine is more likely due to lack of fluid intake and concentrated urine. If a person were to limit fluids in hope of decreasing bladder pain and frequency it would result in dark urine. It would not however, be directly due to the cystitis.
Possibly. Interstitial cystitis is caused by a breakdown in the gag layer of the bladder lining. Sometimes you can get a little bleeding in the huhner's ulcers that are sometimes see with i.C. But usually urine color is related to the concentration of the urine and the byproducts of the food that has been digested. Really concentrated urine will be darker than real dilute urine.