Gold nanoparticles color vary as their size vary. In general, nanoparticles smaller than 30 nm are ruby red, up to 100 nm are pink and larger appear darker in color. During storage, change in color may indicate stability problem i.e. aggregation of particles.

So we can say when the color is appear darker following stability  changed and their size became bigger.

Yes, when they aggregate to form bigger lumps, they become darker,

Its not always tru.

You can take help of UV VIS spectrophotometer. blue and red type of shift are observed for increase and decrease in size.

Blue shift = decrease in size

Red shift = increase in size

Thats very interesting from you Akrema Aki. Can you please refer me to literature.

http://eng.thesaurus.rusnano.com/wiki/article737

http://50.87.149.212/sites/default/files/nanoComposix%20Guidelines%20for%20UV-vis%20Analysis.pdf

Thanks a lot dear Akrema.

Agree with Aki. Dear Nejad change in colour is due to the change in size of your nanoparticles

As the shape and size of the nanoparticles change the color of the solution will change. Aggregation can also lead to color changes whereby red changes to blue. To determine if color change is related to size and shape you need to perform UV-Vis. If from UV-Vis absorption you have one peak(around 520 nm) and the wavelength red-shifts with a decrease in intensity overtime the color is related to aggregation and subsequent size changes for spherical particles.

Do you mind sharing your reference? Ndifreke Usen

Article Sonochemical Synthesis of Porous Gold Nano- and Micro-partic...