Lossless color compression
Without knowing the video properties, my generic answer is

ffmpeg -i in.mp4 -vf geq=lum='p(X,Y)':cb=128:cr=128 out.mp4


This will reset the color information, but pass the luma plane intact. Add -crf 0 for truly lossless re-encoding.



This is the faster way,

ffmpeg -i in.mp4 -vf extractplanes=y,format=yuv420p out.mp4


The luma plane is extracted, and then neutral chroma planes are added in the pixel format conversion. This 2nd step is added because otherwise ffmpeg will auto-convert pixel format to yuv444 which isn't widely compatible.