This is the product to use, Discrete, by Restoria. It's not a dye but a restoring cream. I have used it for 15 years and no one ever realizes my 'naturally' dark hair, at 78, is not natural.
Start applying it 3 times a week, reducing as the color changes. No mess, no fuss. Available in Thailand beauty shops, supermarkets. I get mine in Big C Extra in Pattaya.