I will first motivate axions and axion-like particles (ALPs) in general, with new physics developments. I will then discuss the model-independent leading effective interactions between the SM fields and a generic ALP. Two frameworks for electroweak symmetry breaking are considered: linear and non-linear. For the latter case, the basis of leading effective operators is determined. For both scenarios, associated novel signals at colliders involving gauge bosons or the Higgs boson are determined and analyzed. In the second part of the talk, I will present a model with an enlarged color sector which solves the strong CP problem via new massless fermions. QCD colour is unified with another confining group with a large confining scale. Small size instantons provide an extra source of masses for axions, which become heavy. Massless sterile fermions in the low energy spectrum are instead a singular characteristic. The axion scale may be near the TeV region which results in QCD-coloured scalars observable at colliders. This model naturally enlarges the axion parameter space which solves the strong CP problem, well beyond that of invisible axion models.
