ISO by itself affects a photo by making it brighter. That is all.
It does not add any noise or grain (except for an inperceptible amount introduced by the amplifier). The ISO setting amplifies the analogue signal before it is digitised. This actually reduces noise you'd see compared to amplifying after digitisation (as this would amplify the digitisation noise also).
Raising the ISO setting with the camera set to a program exposure mode will cause the shutter time and/or aperture to be reduced, causing a darker photo (which is made up for by the fact that ISO makes the photo brighter). This is where the association with ISO and noise comes from.
Reducing the amount of light coming through the lens increases noise. Photons are emitted randomly by a lightsource, if you capture many photons the randomness evens out, collect fewer photons and the randomness shows up.
With this in mind, the noise introduced by lack of light affects each colour channel differently. The blue channel is the noisiest as the blue filters in the bayer matrix take out more light than the red and green. Also there are twice as many green pixels as red and blue which evens out the noise. Finally blue is also less common in nature (hence the reduced number of blue filters) so in a particular scene you will capture less blue light so the blue channel will appear noisiest, especially if you shift the colours in an image, which is akin to digitally amplifying the noise in the blue channel.
The noise will also show up more in shadow areas as these naturally reflect less incoming light. If you are working in really low light (and compensating by raising the ISO a lot) then you can see significant noise in the highlights also.