There is using dandruff shampoo to fade colour, there is the vitamin C method and dish soap washes just to name a few! Another technique for those who change colours often is to use a colour wheel to gauge what to colour and results when you don't want to strip hair.
Colour wheels are used in makeup, art and yes even hair. So here is how it works...
Let's use green as our starting point for colour. Now if we went for red those opposing colours would cause a muddy mess when put on top of each other. If you want to achieve red eventually and have fun along the way here is what you do.
We want to move to the left or right of green for best results over our green. So we will get either yellow green or blue green hair. (Sometimes you can move 2 places to the left or right but only if your colour is high depositing)
Next time you colour you can move another place to your chose. Side on the wheel towards orange or violet. As you go you will eventually reach red. This is great for those who play with their colour often and don't feel the need to strip colour.
The colour wheel is also a great way to select complimentary colours when using mutiple shades etc.
If you would to see more posts on hair colour and tips and tricks please let me know in the comments :)