complexing agent is not necessary for the formation of colorless crystals of the dye former but is incorporated to allow the tuning of the melting point of the colorless crystals the complexing agent should not interfere unduly with the equilibrium between colored and colorless forms of the dye former in the amorphous state too strong a complex between the colorless form of the dye former and the complexing agent may lead to a faint color in the amorphous state an actual chemical reaction such as a deprotonation between the complexing agent and the dye former may lead to an inability to form the colorless form of the dye former at all or may affect the color of the dye formed from the dye former these requirements limit the basicity of the complexing agent to a narrow range that depends upon the choice of dye former in the present invention specific preferred dye formers bear a phenolic group which forms a hydrogen bond with the complexing agent complexes formed between phenolic and amino compounds are well known and are discussed for example in u201c equilibria of bisphenol complexation with pyridine in acetonitrile solutions u201d by titov e v et al zhurnal obshchei khimii 1993 63 8 pp 1869 71 u201c hydrogen bonding based thermochromic phenol amine complexes u201d by mizutani t et al j phys org chem 1998 11 10 pp 737 742 and u201c hydrogen bond equilibria of phenol pyridine in cyclohexane carbon tetrachloride and benzene solvents u201d by spencer j n et al j phys chem 1987 91 6 pp 1673 4 furthermore u s pat no 4 097 288 discloses that certain phenolic or amino compounds readily form co crystals with hydrogen bonding acceptors or donors respectively the complexes of the invention are preferably formed by one.