This is using the April 2020 release of PVRTexToolCLI (Build 4.23) and GUI (Build 20.1) on macOS 10.15.6. The samples shown are ASTC4x4 encoded 4x4 textures and I’m using astcenc.
I’m only able to visually verify this content is correct off the preview thumbnail (which mostly looks like the texture in the upper left), since KTX thumbnails in macOS don’t handle sRGB. PVRTexToolGUI gives totally different results from Preview for the PNG and KTX file. My display is set to sRGB as well.
DCM (digital color management tool) says the following for red:
png 64 13
ktx 136 64
Your website doesn’t allow upload of KTX format files, so this is the command to process that file.
PVRTexToolCLI -f ASTC_4x4,UBN,sRGB -i AttribSimple.png -o AttribSimple.ktx
sRGB handling should purely affect mipmap generation where srgb to linear conversion occurs at higher than 8-bit depth, mips are filtered, converted back to 8-bit sRGB, and then sent to encoder. Ideally higher bit depth linear pixels would be sent to encoding, and endpoints converted back to sRGB after fitting the point cloud, but that is more involved and difficult with 565 encodings like BC1-3.