Hi! Yesterday I bought the 5mm yoga mat in dark green (solid color, no pattern). I laid it on my floor to air out and try the mat. I did an extended child's pose and immediately there was an oil spot from my forehead (I don't have particularly oily skin so this didn't even cross my mind). I wiped it with a little dish soap and water but this morning it's still there... any tips? I'm bummed! I thought I got a dark enough color. 