Hi, Thanks for writing in. There is no need to get worried if your tests for all STDs are negative. It can be a fungal infection or mild bacterial infection. A fungal cream usually takes a week or two weeks of application to show any visible change but the itching disappears in 2 to 3 days. The other possibility is a mild infection of the foreskin (if uncircumcised) or the glans. This might require a short course of antibiotics. Many times color changes in the tip of penis without any symptoms can be a normal occurrence. A dermatologist will confirm your condition and give you more specific treatment. Please maintain genital hygiene and wear cotton inner clothes if you sweat a lot.