I would recommend keeping on for entire 4 hours if you are not noticing results right away.
We are having great success with the Ultra One. Did you purchase your Ultra one recently or earlier when we released the product in September? We've made some revisions for all orders after Sept 20th which should result in darker color.
We do have options available if you wish to have the
solution darker, give us a call at 1-888-495-6064.
Thank you!