  We study systems in which both long-ranged van der Waals and critical Casimir
interactions are present. We study the interplay between these forces, as well
as the {\it total} force (TF) between a spherical colloid particle and a thick
planar slab, and between two spherical colloid particles. We do that using
general scaling arguments and mean-field type calculations utilizing the
Derjaguin and the surface integration approaches. They both are based on data
of the forces between two parallel slabs separated at a distance $L$ from each
other, confining the fluctuating fluid medium characterized by its temperature
$T$ and chemical potential $\mu$. The surfaces of the colloid particles and the
slab are coated by thin layers exerting strong preference to the liquid phase
of the fluid, or one of the components of the mixture, modeled by strong
adsorbing local surface potentials, ensuring the so-called $(+,+)$ boundary
conditions. On the other hand, the core region of the slab and the particles,
influence the fluid by long-ranged competing dispersion potentials. We
demonstrate that for a suitable set of colloids-fluid, slab-fluid, and
fluid-fluid coupling parameters the competition between the effects due to the
coatings and the core regions of the objects involved result, when one changes
$T$, $\mu$ or $L$, in {\it sign change} of the Casimir force (CF) {\it and} the
TF acting between the colloid and the slab, as well as between the colloids.
This can be used for governing the behavior of objects, say colloidal
particles, at small distances, say in colloid suspensions for preventing
flocculation. It can also provide a strategy for solving problems with
handling, feeding, trapping and fixing of microparts in nanotechnology. Data
for specific substances in support of the experimental feasibility of the
theoretically predicted behavior of the CF and TF have been also presented.
