A sensitive color test for uronium nitrate (UN) was recently developed. It is based on the formation of a red dye in the reaction between p-dimethylaminocinnamaldehyde (p-DMAC) and UN under neutral conditions. A similar reaction with p-dimethylaminobezaldehyde (p-DMAB) produces a yellow dye. This paper describes how sizeable crystals of both dyes were obtained and measured. Crystallographic measurements show that both dyes are mono-ureido nitrate salts, which are protonated on one of the urea nitrogen atoms. In their 1956 article, Cline and Fink, argued for a protonated Schiff based structure for the product of urea and p-DMAB. Their experimental proof of this theory was only partial, based mostly on microanalysis. In a more recent article, Hussain and Shaukat reported a free-base structure for the colored product of urea with p-DMAC. The crystallographic data from the current study confirm the protonated Schiff base structure for both dyes, thus supporting Cline and Fink's assumption. Further, examination of the bond lengths in the two dyes shows that C(2)-C(3) and C(5)-C(6) are the shortest bonds in both aromatic rings, indicating a higher degree of double bond, which shows that the quinoid form prevails in the crystal lattice. 2 tables, 10 figures, and 15 references