I have a grey coloured discoloratio on my skin just under my breast bone between my breasts. It is not itchy or red or painful in any way.

Multiple causes . This may be caused by a number of different issues. Post-inflammatory hyperpigmentation is darkening of the skin that can occur after minor injury such as rubbing or after a minor rash resolves. Seborrheic keratoses are benign grayish to brownish age or wisdom spots that grow in women commonly in this area. See a dermatologist to evaluate the cause of the discoloration in your case.