PNG vs JPG color off by 1

[af_luther]

I am automating a process in Python and I am loading a PSD into GIMP, changing a fill layer's color, and then exporting the result to JPG and PNG.
I am setting the fill layer to a light grey #3F3F3F (63,63,63). e.g. with

Quote:gimp_color = gimpcolor.RGB(int(rgb[0]), int(rgb[1]), int(rgb[2]))
pdb.gimp_context_set_foreground(gimp_color)
mylayer.fill(FILL_FOREGROUND)

The JPG output has that same color as expected (#3F3F3F).
The PNG output has changed that area to #3E3E3E (62,62,62).
Some other color areas I set are also off by 1 in the PNG, but not the JPG. Out of 6 different fill layers, 4 are correct and 2 are off.
I would like to correct this. In fact, I was thinking PNG would be better because I do have some artifacts in the JPG where the compression changes things a bit and the PNG should be lossless.

I noticed these messages in the output which I thought might be related:

Quote:gimp_color_transform_new: using babl for 'sRGB IEC61966-2.1' -> 'GIMP built-in sRGB'
gimp_color_transform_new: using babl for 'GIMP built-in sRGB' -> 'sRGB IEC61966-2.1'
gimp_color_transform_new: using babl for 'sRGB IEC61966-2.1' -> 'GIMP built-in sRGB'
gimp_color_transform_new: using babl for 'GIMP built-in sRGB' -> 'sRGB IEC61966-2.1'
gimp_color_transform_new: using babl for 'GIMP built-in sRGB' -> 'sRGB IEC61966-2.1'
void gimp_gegl_convert_color_profile(GeglBuffer*, const GeglRectangle*, GimpColorProfile*, GeglBuffer*, const GeglRectangle*, GimpColorProfile*, GimpColorRenderingIntent, gboolean, GimpProgress*): converting buffer took 0.0109 seconds

Any tips on how I can avoid this issue of the color values in the PNG export being slightly off from what I set?
The two places I am seeing the "wrong" value in the PNG is (1) loading it into Paint.net and (2) in a WebGL shader that uses it as a texture. In both of those programs the values I am seeing are the same.