The effect is named after John Ridley Stroop, who published the effect in English in 1935 in an article entitled Studies of interference in serial verbal reactions that includes three different experiments.  However, the effect was first published in 1929 in German, and its roots can be followed back to works of James McKeen Cattell and Wilhelm Wundt in the nineteenth century.  In his experiments, J. R. Stroop administered several variations of the same test for which three different kinds of stimuli were created. In the first one, names of colors appeared in black ink.
In the second, names of colors appeared in a different ink than the color named. Finally in the third one, there were squares of a given color.  In the first experiment, 1 and 2 were used. The task required the participants to read the written color names of the words independently of the color of the ink (for example, they would have to read “purple” no matter what the color of its ink was). In the second experiment, stimulus 2 and 3 were used, and participants were required to say the color of the letters independently of the written word with the second kind of stimulus and also name the color of the dot squares.
If the word “purple” was written in red, they would have to say “red”, but not “purple”; when the squares were shown, the participant would have to say its color. Stroop, in the third experiment, tested his participants at different stages of practice at the tasks and stimulus used in the first and second experiments, to account for the effects of association.  Stroop identified a large increase in the time taken by participants to complete the color reading in the second task compared to the naming of the color of the squares in experiment 2 while this delay did not appear in the first experiment.
Such interference was explained by the automation of reading, where the mind automatically determines the semantic meaning of the word (it reads the word “red” and thinks of the color “red”), and then must override this first impression with the identification of the color of the word (the ink is a color other than red), a process that is not automatized.  Unlike researchers performing the Stroop test that is most commonly used in psychological evaluation, J. R Stroop never compares the time used for reading black words and the time needed for naming colors that conflicted with the written word.
Experimental findings Stimuli in Stroop paradigms can be divided into 3 groups: neutral, congruent and incongruent. Neutral stimuli comprise those in which only the text (similarly to stimuli 1 of Stroop’s experiment), or color (similarly to stimuli 3 of Stroop’s experiment) are displayed.  Congruent stimuli are those in which the ink color and color name refer to the same concept (for example the “red” word written in red). Incongruent stimuli are those in which ink color and concept differ. 6] Three experimental findings are recurrently found in stroop experiments.  A first finding is semantic interference, consisting of the fact that naming the ink of neutral stimuli is faster than in incongruent conditions. It is called semantic interference since it is usually accepted that the relationship in meaning between ink color and word is at the origin of the interference.  Semantic facilitation defines the finding that naming the ink of congruent stimuli is faster than with neutral stimuli.
The third finding is that both semantic interference and facilitation disappear when the task consists in reading the word instead of naming the ink. It has been sometimes called Stroop asyncrony, and has been explained by a reduced automaticitation when naming colors compared to reading.  In the study of interference theory, the most commonly used procedure has been similar to Stroop’s second experiment in which subjects are tested on naming colors of incompatible words and of control patches; however the first experiment (reading words in black versus incongruent colors) has received less interest.
In both cases, the interference score is expressed as the difference between the times needed to read each of the two types of cards.  Usually lists of stimulus are used, but time measures for individual words permit more control on research variables.  Rather than naming or reading stimuli aloud, subjects have also been asked to sort stimuli into categories.  Different characteristics of the stimulus such as ink colors or direction of words have also been systematically varied.  None of all these modifications eliminates the effect of interference.