amorphous form or in solution on the other hand each molecule may explore its whole conformational and isomeric space and only a small proportion of the population of individual molecules may at any one time exhibit the particular conformation or isomeric form adopted in the crystal in the present invention at least one of the molecules can equilibrate between a colored and a colorless form both of which are present in the amorphous state but only one the colorless form is present in the crystalline state this potentially colored molecule is hereinafter referred to as the u201c dye former u201d two problems commonly occur in designing crystalline colorless color forming compounds having the desired properties firstly it may turn out to be impossible to crystallize the colorless tautomeric form of the dye former for example many of the rhodol type dye formers of the present invention cannot be readily crystallized in colorless form secondly the colorless form may be able to be crystallized but may exhibit a non ideal melting point to change the melting point would require a complete redesign of the dye former a long and tedious process the presence of co crystallizing molecules referred to herein as complexing agents together with the dye former may alleviate either of the problems discussed above in some cases the complexing agent facilitates the formation of colorless crystals from the dye former in other cases the complexing agent is not necessary for the formation of colorless crystals of the dye former but is incorporated.