Wavelength filter with OpenCV
In general this is not possible, since you do not have full spectral information for each pixel, but only coordinates in some low-dimensional color space (i.e. RGB).

If your image data is in RGB, then the best you can do is to simply take the green color channel (which will be most responsive to 550 nm) and throw away the red and blue.  

Depending on the actual response of your RGB channels, you might be able to better estimate the 550nm contribution via a linear combination of red and green (see Spectral Sensitivity).