My hair is already coloured. Can I use this to refresh my colour or cover regrowth, and what precautions should I take?

This product can be used to update regrowth or deepen an existing cool hair colour, as mentioned in the source material. For covering regrowth, apply the colour mixture only to the new growth at the roots first, leaving it for most of the development time, then work through the lengths for the final few minutes to refresh the ends. This prevents over-processing previously coloured hair. If your goal is to deepen or change your overall colour, apply it from roots to ends. The most critical step here is the strand test. The product information specifically recommends it for recently lightened or chemically processed hair. A strand test will show you how the Metallic Graphite interacts with your current colour, revealing the final tone, coverage, and whether your hair's porosity causes it to develop darker or cooler than expected. This is essential for avoiding unwanted results when layering colour over colour.