A pink color indicates an inflammation of the scalp - either due to an allergic reaction or due to infection of the hair follicles or scalp. The infection can be either fungal or bacterial. Both these conditions will also cause hair loss.
Since you itch when you go a few days without shampoo, I suspect that you have a mild fungal infection - the skin of the scalp tends to shed in flakes in fungal infection causing itching , soreness and inflammation. These flakes of scalp are what is known as dandruff. In a mild case od dandruff, you may not notice any flakes falling off, but piling up of the flakes on the scalp will cause itching.
You will need to use antifungal shampoos containing Ketoconazole, available as Nizoral(OTC). Another good shampoo is called Selsun - it contains selenium sulphide and is quite effective against fungal infections that cause dandruff.
Also take Motrin 500mg after food to decrease the inflammation.
A mild allergic reaction to the hair color can also cause similar symptoms. You can try changing the brand of color and see if that helps.
If the soreness and pinkness of the scalp does not decrease in 1 month, please cosnult a dermatologist.
I hope this helps.
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