Yes we read it in the red channel.
Ok now i understand, I thought the same as @shaderbytes at first.
Well, it should work with most channels, except the normalmap because we "normalize" the texture.
Sometimes, people put the diffuse texture into the normalmap channel and thus, the model turns "blueish" (because of normalization).
Note that soon, we'll stop doing the normalization if the texture is used in another channel than the normalmap.
However, concerning the colorspace, if you explicitly change the colorspace, it is for the texture, not the channel.
So if you really want to have a texture with 2 different colorspaces... let's say a roughness (linear) data and (srgb) diffuse data in it, you should keep the colorspace to "auto".
"Auto" infers the colorspace depending of the channel.
Actually, as @renatoaruffo said, I strongly advise you to keep the colorspace to "auto", especially with normalmap