Over the years, I have tried anything that exists on the planet, with only average success, until in desperation, I started colouring the Cs red and the Fs blue.
I have one of Bonnie Mohr’s music books called “Colorful Christmas Carols for the Harp”. She has all the C’s & F’s colored in on the small harp arrangements. It has helped me tremendously when I was starting to learn the bass clef.