If you need to detect colors in your future project, whether that’s for your Lego Mindstorm or whatnot, you might want to take a look at this nifty color sensor circuit and save a lot of headache and time.

Color sensing depends on measuring the intensity of light at different wavelengths. A spectrometer does this by splitting the light with a prism or diffraction grating into its component wavelengths. However, spectrometers are complex and too difficult to build. Another approach measures the light intensity in three primary color bands: red, green, and blue. Then calculates the color or hue of the light in software.
Hue is a single number that describes the overall color of a light. It ranges from 0 to 360, much like the degrees of angle around a circle. In the case of hue: 0 is red, 120 is green, 240 is blue and 360 is back around to red again. Intermediate colors have hues like 60 for yellow, 180 for cyan and 300 for magenta. The calculation of hue from red, green and blue values is a simple algorithm described later.
After substantial experimentation, I found that the blue and green plates were too pale for use individually. Stacking two green and three blue plates created enough filter density for good color measurement. The actual color spectrum of the resulting filters can be seen by photographing sunlight shining through them with a diffraction grating

via hackedgadgets

More resource on color sensor circuits:

• Color Sensor Circuit Diagram
• Color Sensor from LED and Op Amp
• Another Color Sensor Circuit Diagram
• Color Sensor Patent