Basics of Spectral Reflectance

In 1666, Sir Isaac Newton observed that white light is made up of the spectrum of colors. Therefore, giving us the understanding of the Additive Color Mixing. That is when you combine primary lights (blue, green and red) in their highest intensity and equal amounts this results in white light.

When looking at spectral reflectance curves produced from different objects, it can be seen that:

WHITE
White is 100% reflectance at all visible wavelengths from 400 to 700 nm.

BLACK
Black is 0% reflectance at all visible wavelengths from 400 to 700 nm.

GRAY
Therefore, "straight lines" with equal reflectance at all visible wavelengths from 400 to 700 nm are shades of gray. If the reflectance is nearer to 100%, this is a light gray. Likewise, if the reflectance is nearer to 0%, this is a darker gray.

COLORS
Not until there is deviation from this "straight line" do we see various colors.

Blue reflectance curve

Here we see a blue reflectance curve. It has reflectance in the blue region of the spectrum (400-500 nm) and very little or no reflectance in the green (500-600 nm) and red (600-700 nm) regions of the spectrum.

Green reflectance curve

Here we see a green reflectance curve. It has reflectance in the green region of the spectrum, but very little or no reflectance in the blue and red regions of the spectrum.