Demonstration of a colorimetric approach for nucleic acid detection represents an attractive educational experiment for chemistry undergraduate students in the time of coronavirus pandemic. Herein, a rapid and vivid detection method that visualizes the presence of a specific DNA sequence is described. The plasmon resonance of gold nanoparticles (AuNPs) (~20 nm in diameter) endows a red color to DNA-functionalized gold nanoparticles (DNA-AuNPs) in a colloidal solution. Simply upon addition of a sample containing the complementary DNA sequence, the DNA-AuNPs dispersed in a strong ionic solution can spontaneously aggregate in minutes and turn to be purple in color, which can be detected using UV-visible spectroscopy as well as the naked eye. In contrast, the DNA-AuNPs remain highly dispersed as a reddish colloid even at high ionic strength when the DNA sequence is terminally mismatched or overhung. According to the teaching experience, this DNA testing experiment can be accomplished by students in pairs or groups under instruction in a three-day lab session. Given the rapid and fail-proof DNA testing, the testing method is also well-suited for hands-on introduction of analytical chemistry and nanotechnology to chemistry freshman and high school students.