A radiation-curable fiber optic coating composition for an inner primary coating includes a coloring agent, preferably a dye or a dye precursor, compatible with the fiber of the fiber optic and capable of imparting a pre-selected color to the inner primary coating, or another coating. The coloring agent can be a reactive dye. Any of the dyes preferably is stabilized by a stabiliser in the colored coating layer, or, preferably, in a more exterior layer.