Apply least square fitting to color images downloaded from Wolfram Language Color Schemes.
Top row (poly): polynomial functions of varying degree;
Middle row (trig): functions in the form color(x) = c₀ + c₁⋅x + a₀⋅cos(π⋅x-u₀) + ∑ₖ[aₖ⋅cos(2kπ⋅x-uₖ)];
Bottom row (cosine): functions in the form color(t) = a + b*t + c*cos(d*t+e);
Coefficients for `cosine` are computed numerically and may not be optimal.
Disclaimer: I’m not related to Wolfram in anyway. Wolfram reserves the rights of these color schemes.