  Optical fields with rich and well-developed spatio-temporal structure,
including ultra-short structured light pulses, are essentially
non-monochromatic and contain a continuous spectrum of monochromatic
constituents. However, some substantial features of such fields and physical
mechanisms determining their behavior can be understood based on simplified
models including only two monochromatic paraxial components. We consider
examples of such model beams, their specific spatial and temporal properties as
well as their descriptive abilities for the meaningful characterization of
realistic spatio-temporal light fields. In particular, the proposed model
enables an explicit consistent analysis of the photon-probability distributions
in non-monochromatic fields, which confirms a high degree of coincidence
between the "energy center" and "probability center" of the field.
Simultaneously, particular features of the two-component bi-chromatic paraxial
fields (periodic and rotational character of the longitudinal and temporal
evolution, specific deformations of the propagating-beam transverse intensity
profile, etc.) are inspected using numerical examples.
