Method of coloring an organic material with an asymmetric thioindigoid compound having the formula: ##STR1## wherein R is a hydrogen atom or lower alkyl group; R' is a member selected from the group consisting of a hydrogen atom, halogen atoms, lower alkyl groups, lower alkoxy groups, sulfonamide group or its derivatives, and aryl sulfonate groups or their derivatives; R" is a hydrogen atom or methyl group; and X is an oxygen atom or sulfur atom. Organic materials colored with the compound have a brilliant nuance with characteristic fluorescence.