*2.6. Color Stability of Bi-Layer Films*

In order to measure the stability, the films were kept at 4 ◦C or 25 ◦C at 2-day intervals within 14 days using a portable scanner. The calculation of color changes (Δ*E*) was as follows:

$$
\Delta E = \sqrt{(L - L\_0)^2 + (a - a\_0)^2 + (b - b\_0)^2} \tag{4}
$$

where *L*, *a*, and *b* are the color values of the films at storage time; *L*0, *a*0, and *b*<sup>0</sup> are the initial color values.
