We present a novel psychophysical method for identifying independent directions in color space, measuring the spectral tuning of color mechanisms, and customizing equiluminance settings for individual observers. The stimulus consists of a spatial ramp (a shaded rectangle that is dark on bottom and light on top) surrounded by a field whose luminance level is modulated in time (see demo at www.shapirolab.net ). The ramp appears to be divided at the point of zero contrast (i.e, the point at which the luminance level of the background matches the luminance of the ramp). If the background modulates between two lights of unequal luminance, the divider will slide up and down the ramp; if the two lights have the same luminance, the divider will remain at a fixed level. We present experiments in which the ramp is always achromatic, centered at the mean luminance of the monitor (R, G, B phosphors equal to .5). The mean level of the background is also set to R, G, B = .5, and each phosphor (R alone, G alone, B alone, and all three together) is modulated at 1 Hz. Observers adjust markers on the side of the ramp to indicate the maximum and minimum levels reached by the perceived divider. The observers’ settings are plotted vs. the modulation level of the phosphors. The observers’ settings track linearly the maximum and minimum luminance levels of the different phosphors. The ramp can therefore be read as a gauge indicating the relative contrast in the surrounding field.