A spectrophotometer measures the wavelength of light emitted by a solution. Since different colors emit a different wavelength of light, the spectrophotometer gives a numeric value to a particular color.
In your analysis of your ELISA and Bradford assays, you may can interpret your results in two ways. The first way is just be comparing the color change of your patient sample to the standards in order to estimate the concentration. The second way is to use a spectrophotometer if your lab has one available. By using the spec, you will get a quantatative number for each patient samples. You can then compare this number to your standards in order to get a more accurate concentration.
The Virtual Analytic Laboratory has a video that shows how spectrophotometry works.