A series of poly(4-vinylpyridine)-b-poly(ε-caprolactone) (P4VP-PCL) diblock copolymers have been synthesized and used for the formation of nanostructured materials with tunable colors arising from the association of chromophores with P4VP block in P4VP-PCL. The association of chromophores leads to the bathochromical shifts of charge transfer absorption peaks resulting in the color appearance into visible region. To achieve the formation of well-controlled, nanostructured materials, the phase behavior of the mixtures of chromophore/P4VP-PCL was systematically examined. As evidenced by transmission electron microscopy and small angle X-ray scattering (SAXS), the phase transformation of self-assembled nanostructures can be easily induced by adding chromophores due to the association of 2-methylidenepropanedinitrile in the chromophores with the lone-pair electron of nitrogen in P4VP block (that is the increase on the effective volume fraction of P4VP, as identified by SAXS experiments through one-dimensional correlation function). Accordingly, transparent thin films of chromophore/P4VP-PCL mixtures can be fabricated and the appearance of color in visible region for the mixtures can be tuned by exploiting the bathochromical shifts of charge transfer absorption peaks. Systematic studies with respect to the color tuning by using various stimuli such as temperature, moisture and pH for chromophore/P4VP-PCL thin-film samples were conducted. The dissociation behavior can be easily observed while elevating temperature higher to 160oC. However, no recognized change can be found after dissociation even with temperature lower than ambient temperature; we speculate that the irreversible process in response to temperature is attributed to the diffusion limitation for re-association chromophores with P4VP. By contrast, the color change of chromophore/P4VP-PCL mixtures in response to the pH variation can be sensitive because of the replacement of the protonation between pyridine and H+ with the addition of HCl solution. The color change from transparent to red can also be observed by adding NaOH solution, demonstrating the reversibility of the association and dissociation processes. As a result, by taking advantage of charge transfer, well-defined nanostructured materials resulting from mixing of chromophore and P4VP-PCL offer the possibilities to create stimuli-responsive transparent polymeric materials with tunable color.