Such objects are closely related to amorphic association schemes; these are association schemes for which any merging of classes is again an association scheme.Such an object often leads to an object in the question (and there are quite often many non-isomorphic examples). Here is a link to a survey (published in J.Comb.Th.(A)).
Moreover, for $c\leq 3$ one always gets an amorphic association scheme. For $c=2$ this is trivial, and for $n=3$ this is discussed in Sect. 7 of J.Comb.Th.(A).
PS. See also Strongly regular decompositions of the complete graph.