The execution of the Chen-Kao reaction is simple, needs little practice and limited skills. 
Also, the violet color obtained in the reaction is easy to define.  
For a correct execution, it is important  to  note  that  the  typical  colors  develop  relatively  slowly,  and  that  a  good  color  intensity requires a sample of a few milligrams of the substances tested (i.e., more than what would typically be required for most other tests included in the UN test kits).
Of all ephedrine-related compounds, only ephedrine and pseudoephedrine produce the typical, stable violet color. Other ephedrine-related compounds produce a blue to greenish-blue precipitate. This precipitate could be seen as characteristic for the members of the ephedrine group other than pseudoephedrine and ephedrine itself, hence, the Chen-Kao test appears to show a significant specificity within the ephedrine group. However, it is known from previously published cross-testing work that various pharmaceuticals not related to the ephedrine group may produce similar blue copper complexes.
The two keto-amines, cathinone and methcathinone, initially also produce blue-colored complexes with the Chen-Kao reagent. However, a slow transition of the initial color into yellow, followed by an orange-brown color can be observed with both compounds, thus indicating the instability of the complexes initially formed, and an obvious decomposition of the two compounds under the alkaline conditions of this color reaction.
The results of the solvent extractions, summarized in Table II appear to add little novelty to the results of the original Chen-Kao reactions. However, in cases of doubts, they may serve as confirmatory steps.