Viscoelastic properties of dense colloids are investigated using the stress-fluctuation formalism and equilibrium molecular dynamics simulations. The focus is on a two-dimensional (2D) non-additive Lennard-Jones binary mixture, which is a glass-former. Its behaviour is compared with that of a one-component Lennard-Jones system, which forms a 2D crystal with triangular lattice at low temperatures. The systems are under fixed pressure and temperature conditions. The effects of the mixing, the temperature and the pressure are studied. Calculated functions include the shear modulus relaxation function, the dynamic moduli (storage modulus and loss modulus), the complex compliance and the dynamic viscosity. Both the liquid and the solid (crystal and glass) states are investigated. Qualitative aspects are discussed in the light of simple rheological models.