App compares Lab values of two colors, as a result returns ΔE00, simple as that. When comparing Lab values it is important to know exactly what is compared. What were measurement conditions, device and environment setup. In case of this app light or measurement condition inputs were skipped deliberately as irrelevant in ΔE calculation algorithm.
Parameters kL, kC, kH are equal to 1 by default, this is recommended by norms.
kL - Lightness parametric factor.
kC - Chroma parametric factor
kH - Hue parametric factor