A blue filter will only let blue light pass while it stops light of all other colors. If you take a picture with black & white film but put a blue filter over the camera lens, then the brightest places in the image will be those that received blue light.

If you take pictures with different filters, you'll see different places brighten and darken in each of the three images.

If you then add the black and white slides together with the color filters placed in front of the slide projector, you would reproduce the true color of the object! We do the equivalent of this technique using computers.