Context :

Aims : We studied the influence of convection on the spectral energy distributions , photometric magnitudes , and colour indices of different types of stars across the H–R diagram .

Methods : The 3D hydrodynamical CO ^ { 5 } BOLD , averaged \left \langle \mbox { 3 D } \right \rangle , and 1D hydrostatic LHD model atmospheres were used to compute spectral energy distributions of stars on the main sequence ( MS ) , main sequence turn-off ( TO ) , subgiant branch ( SGB ) , and red giant branch ( RGB ) , in each case at two different effective temperatures and two metallicities , [ \mathrm { M / H } ] = 0.0 and -2.0 .
Using the obtained spectral energy distributions , we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins UBVRI and 2MASS JHK _ { s } , and the medium-band Strömgren uvby photometric systems .

Results : The 3D–1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed \pm 0.03 mag .
Only in the case of the coolest giants located on the upper RGB are the differences in the U and u bands able reach \approx - 0.2 mag at [ \mathrm { M / H } ] = 0.0 and \approx - 0.1 mag at [ \mathrm { M / H } ] = -2.0 .
Generally , the 3D–1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths .
They are also sensitive to the effective temperature and are significantly smaller in hotter stars .
Metallicity also plays a role and leads to slightly larger 3D–1D differences at [ \mathrm { M / H } ] = 0.0 .
All these patterns are caused by a complex interplay between the radiation field , opacities , and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres .
Although small , the 3D–1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures .

Conclusions :