  In chameleon dark energy models, local gravity constraints tend to rule out
parameters in which observable cosmological signatures can be found. We study
viable chameleon potentials consistent with a number of recent observational
and experimental bounds. A novel chameleon field potential, motivated by f(R)
gravity, is constructed where observable cosmological signatures are present
both at the background evolution and in the growth-rate of the perturbations.
We study the evolution of matter density perturbations on low redshifts for
this potential and show that the growth index today gamma_0 can have
significant dispersion on scales relevant for large scale structures. The
values of gamma_0 can be even smaller than 0.2 with large variations of gamma
on very low redshifts for the model parameters constrained by local gravity
tests. This gives a possibility to clearly distinguish these chameleon models
from the Lambda-Cold-Dark-Matter model in future high-precision observations.
