Unwanted tones often appear when lifting hair to a higher level. This is particularly true when lightening dark hair, as it contains a higher amount of pheomelanin.
Pheomelanin is resistant to lifting more than 4 levels, therefore a lightening process is often required. When lifting, the size of the pheomelanin is reduced changing from red to orange, then to yellow before it finally dissolves.
Since brown-black eumelanin dissolves more readily, the pheomelanin shows through more strongly towards the final stages of the lightening process. This at times creates unwanted warm tones. So, getting the mixing ratio and development time correct is crucial to achieving an accurate target result.