  Calculations for processes involving a high multiplicity of coloured
particles often employ a leading colour approximation, where only the leading
terms in the expansion of the number of colours $N_c$ and the number of
flavours $n_f$ are retained. This approximation of the full colour result is
motivated by the $1/N_c^2$ suppression of the first subleading terms and by the
increasing complexity of including subleading colour contributions to the
calculation. In this work, we present the calculations using the antenna
subtraction method in the NNLOJET framework for the NNLO QCD corrections at
full colour for several jet observables at the LHC. The single jet inclusive
cross section is calculated doubly differential in transverse momentum and
absolute rapidity and compared with the CMS measurement at 13 TeV. A
calculation for dijet production doubly differential in dijet mass and rapidity
difference is also performed and compared with the ATLAS 7 TeV data. Lastly, a
triply differential dijet cross section in average transverse momentum,
rapidity separation and dijet system boost is calculated and compared with the
CMS 8 TeV data. The impact of the subleading colour contributions to the
leading colour approximation is assessed in detail for all three types of
observables and as a function of the jet cone size. The subleading colour
contributions play a potentially sizable role in the description of the triply
differential distributions, which probe kinematical configurations that are not
easily accessed by any of the other observables.
