with formula {bl} > 16 and string.format('%s%.2x%s','FF',({bl}-16)*255/50,'00'), the color changes when the battery discharges but does not reset when the battery is charged. I must send the watchface the watch for it to work.
(there is a small error in the formula, the calculation ({bl}-16)*255/50 generates color-coded to 7 characters The good formula. ({bl}-16)*255/84)