  Ground-based high-contrast instruments have yielded reflected light images of
protoplanetary disks. Quantitative measurements of the reflected radiation
provide strong constraints on the scattering dust which can clarify the dust
particle evolution in these disks and the composition of the forming planets.
This study aimed to derive the wavelength dependence of polarized reflectivity
$(\hat{Q}_{\varphi}/I_\star)_\lambda$ for 11 disks, constraining dust
properties and identifying systematic differences. Using ESO archive data from
SPHERE/ZIMPOL and SPHERE/IRDIS instruments, we obtained accurate intrinsic
polarized reflectivity $\hat{Q}_\varphi/I_\star$ values at wavelengths from
0.62$\mu$m to 2.2$\mu$m.
  Polarized reflectivities ranged from $Q_\varphi/I_\star\approx 0.1\%$ to
1.0$\%$, with PSF-corrected values averaging 1.6 times higher than observed.
Accurate PSF calibrations reduced systematic errors to
$\Delta\hat{Q}_\varphi/\hat{Q}_\varphi\approx 10\%$ or less. For each disk, we
derived a polarized reflectivity color $\eta_{V/IR}$ between a visible band
$\lambda<1~\mu$m and a near-IR band $\lambda>1~\mu$m and other wavelength
combinations. Wavelength gradients $\eta$ varied significantly among objects.
Disks around Herbig stars (HD 169142, HD 135334B, HD 100453, MWC 758, and HD
142527) showed a red color $\eta_{\rm V/IR}>0.5$, suggesting rather compact
dust grains. T-Tauri star disks (PDS 70, TW Hya, RX J1615, and PDS 66) were
predominantly gray $-0.5<\eta_{\rm V/IR}<0.5$, with an absence of blue colors
incompatible with porous aggregates. Exceptional red colors for LkCa15 and
MWC758 were attributed to potential extra reddening from hot dust near the
star. Future studies incorporating parameters like fractional polarization
$\langle p_\varphi \rangle$ hold promise for advancing our understanding of
dust properties within protoplanetary disks.
