If the wavelength chosen on a spectrophotometer is not the wavelength at which a solution absorbs maximum light, the measured absorbance will likely be lower than the actual maximum absorbance of the solution. This is because the spectrophotometer measures the intensity of light transmitted through the solution at a specific wavelength, and the absorbance is directly related to the amount of light absorbed by the solution.
When the wavelength selected is not the one at which the solution absorbs maximum light, the amount of light absorbed by the solution will be lower. This can result in a lower measured absorbance value. The measured absorbance at a different wavelength might still provide some information about the solution's properties, but it will not accurately reflect the maximum absorbance of the specific compound or chromophore in the solution.
In the example you mentioned with a blue solution typically absorbing light at 645 nm, if a different wavelength is chosen on the spectrophotometer, the measured absorbance will not represent the maximum absorbance of the solution at 645 nm. Instead, it will provide information about the absorption of the solution at the chosen wavelength. To accurately determine the maximum absorbance of the blue solution, it is important to select the appropriate wavelength at which the solution absorbs the most light, in this case, 645 nm.