Alfred, your question seems to me highly dependent on which of the additional "soft" (flautando & tutti/pianissimo) articulations and patches that Duality Colors has, compared to SSPro, that you would value highly and want to use. Also then arises the question of which of those Colors "soft" extras could perhaps be mimicked to some extent in SSPro by using "Timbre" and "Envelope" controls.
A full list of the articulation/patch specifications for Duality Colors can be found under the "Content" heading in its Product pages, so you can at least compare them with the articulation/patch specifications in the SSPro handbook.
Of course the one giant difference is that Duality Colors can present the larger (Stage A) and smaller (Stage B) subsections individually or together, with the possibility of each subsection's audio being routed and processed separately. This feature alone makes Duality a dream machine for me.
I'd say the question of whether or not these differences would "make sense" for you to buy Duality Colors must be yours alone to judge; difficult though that may be without the opportunity of auditioning Duality Colors side-by-side against your SSPro before buying. I've been able to do this auditioning since installing Colors (I too have SSPro, also SS1 and SSElite among many others) but of course have no way of knowing how someone else would value the various and often subtle differences I'm hearing. Speaking for myself, Duality (regular, colors & sordino; all with additional mics) is now my go-to library - I'm very pleased with all of it!
I don't know if a demo license is available for Colors but it might perhaps be worth you emailing VSL to ask.