ffmpeg fade to specific color values over time
The lut filter expressions don't work with time. geq does.

geq=r='(1-0.7*min((T-S)/D,1))*p(X,Y)':g={the same as r}:b={the same}:enable='gte(t,S)'


S should be your start time in second. D is duration for the change in colours. T and p(X,Y) are variables and should be kept as is.