When mixing you cannot obtain a colour of greater intensity by using a mixture of others. For example, no colour can be added to a mixture of Davy's Gray and Cadmium Lemon to regain the intensity of Cadmium Lemon alone.
Every pigment varies in strength.
Winsor Blue, for example, has a high tinting strength whilst Terre Verte has a low tinting strength. In other words Winsor Blue will have a dominant effect on any mixtures whilst Terre Verte will not have a significant effect in mixtures. Care is required in colour mixing to avoid the strong colours over-dominating the paint surface. Strong colours can be controlled by adding small amounts to the mixture repeatedly until the required hue is reached. High tinting strength colours are often high key whilst low tinting strength colours are often low key.