The instability of convection patterns representing a combination of vertical Rayleigh rolls and horizontal rolls resulting from longitudinal horizontal magnetic field is investigated in an inclined layer of magnetic colloid by experiments and numerical simulations. Visualization of convection patterns is provided by a temperature-sensitive liquid crystal film. The rich spectrum of convection structures is observed against different values of inclination angles and uniform magnetic fields. If the horizontal longitudinal magnetic field is strong enough it extinguishes the convection perturbations along the field direction and stabilizes Rayleigh flows. Observed patterns at about two supercriticalities and with small inclination angles show mostly spatially and temporally chaotic structures.