Hello dear, The darkening of skin on back of arms and hips can be attributed to either exposure to sun or Hyper pigmentation. Improvement can be noticed with the use of sunscreen lotion and management of the underlying cause of the Hyper pigmentation. Kindly consult a Dermatologist for the same. Regarding the issue of increase in height, it is determined by Genetics, Environmental & Nutritional factors and Hormonal factors (most important the Growth Hormone secreted by pituitary gland in brain) An individual stops growing in height after the fusion of the Epiphyses (growth plates) of the long bones, which occurs by the age of 17-21 yrs in males. So...as you are 18 years now, there is still a good chance of further increase in height. An X ray for bone age determination can be taken to look for the fusion of epiphyses. Make sure that you are taking a healthy & balanced diet, exercise regularly & avoid stress. Protein rich diet like soya, groundnut, pulses, etc should be incorporated in your diet plan. One more thing, please do not get influenced by any advertisement or health product which claim to boost your growth hormone & increase height...it may be harmful. Wishing you Good Health. Take care.