One of the problems is that many of the scripts and filters in Avisynth work in different color spaces, so there are a number of color space conversions. I've reduced the number of conversions to the least possible, but there are still a few. The worst of these is the YV12 conversion, which seems to be the main reason for the dimmed colors. I'll merge the chroma channel of SRV10 with the Spline64.
Edit:
I actually found a different way to correct the colors. Here's what frame 4961 and 7218 look like with the correct colors compared to the Spline64: