Please help color conversion.

[bxyhxyh]

#source1
#d=source2
source1.ConvertToYV24()
Spline36Resize(1261,711)
Overlay(source2.ConvertToYV24(),last,x=9,y=1)
ConvertToYV12()

How should I tweak this for minimal loss?

[fvisagie]

Remove as many unnecessary colourspace conversions as possible (e.g. both Spline36Resize() and Overlay() handle just about all colourspaces Avisynth supports)
Inspect your input video and pay attention to the handling of level ranges (the 0-255/0-235/0-240 stuff) in each function call
Pay attention to the handling of colour coefficients (the BT.601/BT.709-type stuff) in each function call
Try to avoid geometric conversion that involves odd numbers

Cheers,
Francois