On top of using safe supplies, a requirement here at Restoring Touch, the two key methods of restoring carpet color through dyeing are spot control and re-coloring, typically applied together. Spot control helps counteract localized discoloration from damaging forces such as sun exposure or water damage by manually applying dye with an expert eye. Recoloring can also be used to adjust colors back to their originals by applying carefully selected dyes to darken the carpet fibers back to the desired colors, though it is necessary to bear in mind that colors cannot be lightened, so some spill discolorations can only be cleaned, while others may need a combination of cleaning and dyeing for the best results.
In the event that carpet cleaning does bring your carpet back to its original color, that would likely mean that the color itself has faded over time or due to damage. If that is the case, there is still an option that doesn’t include throwing away the carpet: dye it!