Thanks!, i'm developer and design not is my thing :( , that´s the reason to make an app to not use photoshop, i've another app to see through the clothes applying filters and algorithms into the illumination haha https://play.google.com/store/apps/details?id=com.lmsoft.xrayeditor ( sorry for the spam )
I have to add some more info or sample on the page. But's not complicated. Just start download & installing Python: http://www.python.org/download/ For creating the mask, probably the easy way is to convert the image in B/N and the paint over in red. Or paint on a layer, and then saving just it.
Really comprehensive photo editing app I've been using for years now, I prefer it to anything I've found on desktop tbh. https://play.google.com/store/apps/details?id=com.iudesk.android.photo.editor