Hi , SavePixelArray node is saving colors wrong. I was easily able to reproduce by using your node in the BeginPlay event of a level. Here are some color conversions that the node writes:
Given color in [0-1] range -> Saved color in 8bit PNG file
0.0 -> 0
0.25 -> 137
0.5 -> 188
0.75 -> 255
1.0 -> 255
Engine version is 4.11 and I used latest version that you shared via mediafire.