Help with Paraview colormap and screenshot
 

Hi I am new to this Forum and I have been working with Paraview for a couple of months.
I want to export the image of a slice in paraview to a tiff image using "save screenshot" - I don't know if there is a better way to do it.

Before I save the screenshot I check to reescale to data range.
But after saving it I check with ImageJ that the pixel values for the extreme value of my data is not the same one present at the colorbar! For exemple red 255. This happens with any colormap option.

I don't know if "save screenshot" is reliable or if there is something else I should know, like if there is a command in python to extract the matrix of the image.

I am attaching an exemple of one image I have, that I used "rainbow".
The extreme point of my data are -2.233e-1 and 9.222e-1.

Thank you in advance.