Wrong way to do it. We know from psychometrics that forced binaries like this just create junk (people disagree with the question, so just choose a forced answer based on some heuristic for each such question like "closest to my mouse / finger" or "most socially desirable" or "same as last time"). So you aren't measuring what you think when you force choice like this.

If you're going to go with linguistic self-report and a single item, you really want something like an 11-point Likert scale. A smart design might get e.g. a person's rating of "blue-ness vs. green-ness" on an 11-point scale, then determine the optimal cutpoint via e.g. clustering, logistic regression, or some other method, to really get something meaningful.