Recolorization of black and white frames

[VideoOnTapes]

Hi there,

I'm newbie with AviSynth. I use AviSynth+ 64bit and AvsPmod 2.6.3.4.

I'm looking to restore old VHS recordings where some frames are in black and white.
Something similar to what's been done here: https://www.youtube.com/watch?v=iaUsvKDQycg

I figured out how to do it with MergeChroma() :

frame_number_with_good_colors = 128
good_frame= clip.Trim(frame_number_with_good_colors,1)

# Apply MergeChroma for frames with bad chroma (AverageChromaU() < 120)
# Apply MergeChroma for frames in black and white (vPlaneMedian() = 127 or 128)
copy=clip
chromed=MergeChroma(clip, good_frame)
clip=ConditionalSelect(clip,"v_median=vPlaneMedian()"+chr(13) \
+ "av_chromaU=AverageChromaU()"+chr(13) \
+"(v_median==128 || v_median==127)?1:(av_chromaU<120?1:0)", copy, chromed)

What I'd like to do is use the color values from the last frame that is correct.
To do this, I need to store the number of this correct frame, as it has to be reused until a new frame with correct colors is found.

However, I can't store the current_frame value in a variable. Could you help me on this point?

Also, I tried to write this value to a file:

clip = clip.WriteFile("variable.txt", "current_frame")

but it doesn't work for me :

Script error: Invalid arguments to function 'WriteFile'

NB: The video I'm working on (as a test basis) is this one:
https://www.youtube.com/watch?v=rX0r6WbiutU