Chameleon skin has relaxed and excited states.
For a while, people thought color change was caused solely by varying distributions--clumping and dispersion--of pigment under different states.
In 2015, research uncovered another mechanism: structural color.
Structural color is produced when nanoscale structural components refract and reflect different wavelenghts (or colors) of light in different ways.
When feature sizes are on the same magnitude as the wavelength of light, selective interference occurs--certain wavelengths of light interfere destructively (cancel out) and other wavelengths interfere constructively (amplify), resulting in structure-determined color even if the color of the bulk material is clear.
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Diagram demonstrating a mechanism of structure color, thin film interference.
(Image Credit: Chih-hao Chang, North Carolina State University)
Chameleon skin contains a lattice of guanine nano-crystals, the spacing of which is different in relaxed and excited states.

Relaxed

excited

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Microscope image of chameleon skin in the ‘relaxed’ and ‘excited’ states. Scale bars = 200 nm.
(Image by Teyssier et al., taken from Photonic crystals cause active colour change in chameleons. Nature Communications, 2015.)
This change in conformation contributes to the chameleon's color change.
Structural color can also be found in morpho butterflies, damselflies, peacocks, opals, and more.
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Inspired by chameleons, I asked myself: 
If changing the spacing between periodic nanostructures can cause color change, can a nanostructured substrate be used as a colorimetric strain sensor?
And, what if I can incorporate another color-changing mechanism into the nanostructured substrate? Hint: plasmonic nanoparticles
Then, how can I efficiently analyze the properties of the nanomaterial I've created?