We consider a low-density assembly of spherical colloids, such that each is clothed by L endgrafted chemically incompatible polymer chains either of types A or B. These are assumed to be dissolved in a good common solvent. We assume that colloids are of small size to be considered as star-polymers. Two adjacent star-polymers A and B interact through a force F originating from both excluded-volume effects and chemical mismatch between unlike monomers. Using a method developed by Witten and Pincus (Macromolecules 19, 2509(Macromolecules 19, (1986) in the context of star-polymers of the same chemical nature, we determine exactly the force F as a function of the center-to-center distance h. We find that this force is the sum of two contributions F e and Fs. The former, that results from the excluded volume, decays as Fe ∼ ALh −1 , with the L -dependent universal amplitude AL ∼ L 3/2 . While the second, which comes from the chemical mismatch, decays more slowly as Fs ∼ χBLh −1−τ , where τ is a critical exponent whose value is found to be τ ∼ = 0.40, and χ is the standard Flory interaction parameter. We find that the corresponding L-dependent universal amplitude is BL ∼ L (3+τ )/2 . Theses forces are comparable near the cores of two adjacent star-polymers, i.e. for h ∼ hc ∼ aχ 1/τ √ L (a is the monomer size). Finally, for two star-polymers of the same chemical nature (A or B), the force F that simply results from excluded-volume effects coincides exactly with Fe, and then the known result is recovered.