A clear answer. Here’s how.
Computes weighted RGB luma 0.2126R + 0.7152G + 0.0722B, then linearly interpolates between the two selected RGB colours. Original alpha is retained. This is interpolation in display RGB code space, without print separations, ICC colour management or perceptual lightness matching. Supports PNG/JPEG/WebP up to 10 MB and 4 million decoded pixels per file. Browser decoding and single-frame PNG re-encoding remove source metadata/colour profiles; no files leave this device.