  We present a merger-driven evolutionary model for the production of luminous
(LIRGs) and ultraluminous infrared galaxies (ULIRGs) with warm IR colours. Our
results show that simulations of gas--rich major mergers including star
formation, black hole growth, and feedback can produce warm (U)LIRGs. We also
find that while the warm evolutionary phase is associated with increased AGN
activity, star formation alone may be sufficient to produce warm IR colours.
However, the transition can be suppressed entirely - even when there is a
significant AGN contribution - when we assume a single-phase ISM, which
maximizes the attenuation. Finally, our evolutionary models are consistent with
the 25-to-60 micron flux density ratio vs. L_HX/L_IR relation for local LIRGs
and ULIRGs, and predict the observed scatter in IR color at fixed L_HX/L_IR.
Therefore, our models suggest a cautionary note in the interpretation of warm
IR colours: while associated with periods of active black hole growth, they are
probably produced by a complex mix of star formation and AGN activity
intermediate between the cold star formation dominated phase and the birth of a
bright, unobscured quasar.
