Please check the following references:

Int. J. Pure Appl. Sci. Technol., 9(1) (2012), pp. 1-8.

Bull. Korean Chem. Soc. 2006, Vol. 27, No. 11, 1871

The same effects are seen for copper NPs as for gold and Ag NPs. The color is related to the size of the NPs.

Yes. Nanoparticles show colour variation due to change in their dielectric properties and size. Copper nanoparticles also do show this properties.

In addition to size, color may also change during bottom up methods depending upon starting materials. I have also heard about Tandall effect being responsible for different colors depending upon the direction you watch and the light source.

Copper nanoparticle dispersions have absorption spectra that contain two contributions. There is an absorption that originates from interband transitions between d and sp bands as well as an absorption due to  plasmon excitation.

The interband transitions for copper occur in a broad range covering from 200nm up to the blue green region. This contribution is not particle size dependent in any significant way as far ass the color of the particle dispersion is concerned.

The plasmon absorption band for copper occurs in the 550-650nm region. Its magnitude increases linearly with the volume of the particle, and is suffers a small red-shift as the particles grow. Thus these two contributions cause copper nanoparticle dispersions to have colors that vary a little in the brownish-redish range of colors.

For particles larger than about 60nm, light scattering also contributes to the color of the particle dispersion, which acquire a turbid look.

Finally, if the nanoparticles are caused to aggregate, more intense absoeprtion in the red and near infrared can occur due to plasmon excitations of the aggregates. These usally cause noticeable absortion in the red edge of the spectrum, making the nanoparticle dispersion to look green-bluish. Note that now the color can change very much but this is not a size dependence of the nanoparticle spectrum. It is an entirely different phenomenon.  You may find  Bhoren and Huffman's book useful. Also you may look into this publication:

Ultraviolet–visible absorption spectra of the colloidal metallic elements

J. Alan Creighton and Desmond G. Eadon

J. Chem. Soc., Faraday Trans., 1991,87, 3881-3891 

Copper nanoparticle dispersions have absorption spectra that contain two contributions. There is an absorption that originates from interband transitions between d and sp bands as well as an absorption due to  plasmon excitation.

The interband transitions for copper occur in a broad range covering from 200nm up to the blue green region. This contribution is not particle size dependent in any significant way as far ass the color of the particle dispersion is concerned.

The plasmon absorption band for copper occurs in the 550-650nm region. Its magnitude increases linearly with the volume of the particle, and is suffers a small red-shift as the particles grow. Thus these two contributions cause copper nanoparticle dispersions to have colors that vary a little in the brownish-redish range of colors.

For particles larger than about 60nm, light scattering also contributes to the color of the particle dispersion, which acquire a turbid look.

Finally, if the nanoparticles are caused to aggregate, more intense absoeprtion in the red and near infrared can occur due to plasmon excitations of the aggregates. These usally cause noticeable absortion in the red edge of the spectrum, making the nanoparticle dispersion to look green-bluish. Note that now the color can change very much but this is not a size dependence of the nanoparticle spectrum. It is an entirely different phenomenon.  You may find  Bhoren and Huffman's book useful. Also you may look into this publication:

Ultraviolet–visible absorption spectra of the colloidal metallic elements

J. Alan Creighton and Desmond G. Eadon

J. Chem. Soc., Faraday Trans., 1991,87, 3881-3891