[phymatter]

is it possible to predict the colour of a salt from the colour of its constituent ions or otherwise ?

[Schrödinger]

Yes. This comes handy when you're dealing with transition metal salts like Ni, Cu, Mn, Co, Cr etc. You can take a good guess at the cation.

For s-block metals, flame test should do the trick. Each cation has a distinct flame.

However, it is not always possible to predict colour based upon one ion alone.

[phymatter]

If we know both the ions then can we always tell the colour of the salt

[Schrödinger]

If you know both ions, you know the salt!!
You can always look up its colour in the literature.