An aqueous composition comprising a colour former, a developer, and a binder, wherein (a) the weight ratio between the developer and the colour former is in the range of from 1 to 5; (b) the binder comprises an acrylate binder, comprising acrylic acid and styrene, and at least one additional monomer selected from the group consisting of α-methyl styrene, ethyl acrylate and 2-ethylhexyl acrylate; and (c) the colour former and/or the developer are present in an un-encapsulated form, wherein the developer is a sulfonylurea derivative of the formula R 1 -S 2 -NH-X-NH-A-B-R 2 wherein R 1 is unsubstituted or substituted phenyl, naphthyl or C 1-20 -alkyl, X is a group of the formula - C -, -C - or -C -, A is unsubstituted or substituted phenylene, naphthylene or C 1-12 alkylene, or is an unsubstituted or substituted heterocyclic group, B is a linking group of formula -O-SO 2, -SO 2 -O-, -NH-SO 2 -, -SO 2 -NH-, -S-SO 2, - O-CO-, -O-CO-NH-, -NH-CO-, -NH-CO-O-, -S-CO-NH-, -S-CS-NH-, -CO-NH-SO 2 -, -O-CO-NH-SO 2 -, -NH=CH-, -CO-NH-CO-, -S-, -CO-, -O-, -SO 2 -NH-CO-, - O-CO-O- and -O-PO-(OR 2) 2, and R 2 is unsubstituted or substituted aryl or benzyl or C 1-20 -alkyl, with the proviso, that, if B is not a linking group of formula -O-SO 2 -, R 2 is unsubstituted or substituted phenyl, naphthyl or C 1-8 -alkyl and that, if B is -O-, R 2 is not alkyl, wherein R 1 as phenyl or naphthyl is unsubstituted or substituted by C 1-8 -alkyl, C 1 - 8 -alkoxy or halogen, and wherein R 1 as C 1-20 -alkyl is unsubstituted or substituted by C 1-8 -alkoxy or halogen, wherein A as a phenylene or naphthylene group is unsubstituted or substituted by C 1-8 -alkyl, halogen-substituted C 1-8 -alkyl, C 1-8 -alkoxy-substituted C 1-8 -alkyl, C 1-8 -alkoxy, halogen-substituted C 1-8 -alkoxy, C 1-8 -alkylsulphonyl, halogen, phenyl, phenoxy or phenoxycarbonyl, and wherein R 2 as aryl is phenyl or naphthyl which are unsubstituted or substituted by C 1-8 -alkyl, halogen-substituted C 1-8 -alkyl, C 1-8 -alkoxy-substituted C 1-8 -alkyl, C 1-8 -alkoxy, halogen-substituted C 1-8 -alkoxy or halogen, wherein R 2 as benzyl can be substituted by the substituents given for R 2 as phenyl or naphthyl, and wherein R 2 as C 1-20 -alkyl is unsubstituted or substituted by C 1-8 -alkoxy, halogen, phenyl or naphthyl. (a) the weight ratio between the developer and the colour former is in the range of from 1 to 5; (b) the binder comprises an acrylate binder, comprising acrylic acid and styrene, and at least one additional monomer selected from the group consisting of α-methyl styrene, ethyl acrylate and 2-ethylhexyl acrylate; and (c) the colour former and/or the developer are present in an un-encapsulated form, R 1 is unsubstituted or substituted phenyl, naphthyl or C 1-20 -alkyl, X is a group of the formula - C -, -C - or -C -, A is unsubstituted or substituted phenylene, naphthylene or C 1-12 alkylene, or is an unsubstituted or substituted heterocyclic group, B is a linking group of formula -O-SO 2, -SO 2 -O-, -NH-SO 2 -, -SO 2 -NH-, -S-SO 2, - O-CO-, -O-CO-NH-, -NH-CO-, -NH-CO-O-, -S-CO-NH-, -S-CS-NH-, -CO-NH-SO 2 -, -O-CO-NH-SO 2 -, -NH=CH-, -CO-NH-CO-, -S-, -CO-, -O-, -SO 2 -NH-CO-, - O-CO-O- and -O-PO-(OR 2) 2, and R 2 is unsubstituted or substituted aryl or benzyl or C 1-20 -alkyl, with the proviso, that, if B is not a linking group of formula -O-SO 2 -, R 2 is unsubstituted or substituted phenyl, naphthyl or C 1-8 -alkyl and that, if B is -O-, R 2 is not alkyl, wherein R 1 as phenyl or naphthyl is unsubstituted or substituted by C 1-8 -alkyl, C 1 - 8 -alkoxy or halogen, and wherein R 1 as C 1-20 -alkyl is unsubstituted or substituted by C 1-8 -alkoxy or halogen, wherein A as a phenylene or naphthylene group is unsubstituted or substituted by C 1-8 -alkyl, halogen-substituted C 1-8 -alkyl, C 1-8 -alkoxy-substituted C 1-8 -alkyl, C 1-8 -alkoxy, halogen-substituted C 1-8 -alkoxy, C 1-8 -alkylsulphonyl, halogen, phenyl, phenoxy or phenoxycarbonyl, and wherein R 2 as aryl is phenyl or naphthyl which are unsubstituted or substituted by C 1-8 -alkyl, halogen-substituted C 1-8 -alkyl, C 1-8 -alkoxy-substituted C 1-8 -alkyl, C 1-8 -alkoxy, halogen-substituted C 1-8 -alkoxy or halogen, wherein R 2 as benzyl can be substituted by the substituents given for R 2 as phenyl or naphthyl, and wherein R 2 as C 1-20 -alkyl is unsubstituted or substituted by C 1-8 -alkoxy, halogen, phenyl or naphthyl.