Inverse Opal

• Color based on sphere size and refractive index, λ = 2d(n_eff² - cos²θ)^1/2, where n_eff = (0.74 n_void + 0.26 n_material)
• Reflective technique so it works exceedingly well in bright sunlight
• Use elastomeric polymer that changes size with mechanical force
• Use gel that reversibly swells and shrinks with application and removal of a voltage