We present a simple analytical mean-field theory for the pair interaction between two colloidal particles, based upon a recent mean-field equation for the depletion thickness 6 which depends on the chain length N, the bulk concentration psi(b), and the solvency chi. Only in the extremely dilute case is (in mean field) the interaction independent of X. At relevant concentrations a better solvency leads for two flat plates to a stronger attraction at contact (similar topsi(b)/delta) and a smaller range of attraction. This range is 2delta(i), where the "interaction distance" delta(i) is in semidilute solutions larger than delta. In the dilute limit, both delta and delta(i) reduce to depletion thickness delta(0) of ideal chains. The pair potential for flat plates can be described by a modified Asakura-Oosawa equation, in which delta(i) takes the place of the original delta(0); this replacement accounts for the concentration and solvency dependence. For the interaction between two spheres of radius a the contact potential is of order alphapsi(b) and nearly insensitive to solvency; again, the range of attraction is smaller for better solvents. For two spheres we calculate the second virial coefficient as a function of concentration and solvent quality and its consequences for the stability of a colloidal dispersion at low colloid concentrations. For relatively short polymer chains the solvent quality hardly matters. For intermediate and large polymer-to-colloid size ratios, increasing the solvent quality leads to an increased miscibility. This implies that the increase in the osmotic pressure for polymers in a good solvent is overcompensated by a decrease of the depletion thickness, leading to weaker interactions.