We explore from both the theoretical and experimental points of view the influence of the laser wavelength on the strain pulses detected in thin transparent layers using the picosecond ultrasonic technique. From a theoretical analysis of the displacement detection involved in such experiments, we predict amplitude and sign changes in the detected signals as the laser is tuned. Experimental results performed on various samples and at different wavelengths are found to be in excellent agreement with the theoretical description. Several applications based on the high sensitivity of the phenomenon are finally proposed.