I have prepared 20 ml of AgNPs in DI water by ps laser, after extracting 10 ml and post-irradiated by same laser parameter for 30 min (just with unfocused laser beam) the color of post-irradiated AgNPs is lighter (lighter yellow color) in comparison with the first 10 ml (As-prepared) logically should be darker because when I post-irradiated for 30 minutes some of DI water evaporated and the concentration should be higher than the first 10 ml.
Maybe laser radiation has encouraged the separation of aggregated particles by creating a charge on them?
you could be making smaller nanoparticles which will have a smaller extinction coefficient so they absorb less light. The extinction coefficient generally tracks the volume of the particle while the absorption of the solution tracks linearly with concentration so your solution will become "lighter" faster as the particles get smaller.
I'd suggest to check spectra of both solutions. In order to relate the observed changes to the concentration, I'd make sure that nothing is happening with the plasmon resonance and no shifts are observed. What you always see (and measure) is a result of light extinction by both absorption and scattering processes. Here you may also consider possible size (of NPs) effects: usually, smaller particles are dominant absorbers while larger particles tend to show scattering contributions (this is what Christopher's already mentioned) .
If you are not changing the conditions of laser ablation (spot size, wavelength,...) only the irradiation time, the size could not change. I suggest that you record the UV VIS spectra of the GNP and check if there is a change in wavelength of the plasmon resonance peak, a change will be related with a change in the particle size and as consequence in color.
Similar was my question, may be the answers help you :)
https://www.researchgate.net/post/What_is_meant_by_different_colours_of_Silver_NPs
you can check both the solutions using the uv absorption spectrometer and with the particle size analyzer (DLS) and verify the exact size of the nanoparticle.
Sundarrajan Parani rightly observed , very nice answer ,
Colour of the Ag NP is dependent on the particles size and only the intensity you can find different with concentration.
I agree with Saurav. I've also tried to synthesize Ag NP in organic phase, and the color of nanoparticle solution changes if particle size changes or particles aggregate together.
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