You can animate a Color parameter. The track is represented by a color gradient.
When adding or editing a key, the Color window allows you to choose the color of the track.
Some parameters only accept integer values. For example, the Index parameter of a Select node accepts only integers between 1 and the number of inputs of the node.
When a track parameter only accepts integers, the curve of this track is displayed with discontinuities to illustrate the different values that the parameter can have.
By default, for the Linear curve type, if a parameter changes from one integer to the next, for example from 1 to 2, the value changes in the middle of the two keys.
To change at the key, you should switch to the Flat then jump curve.
See Changing the type of curve between two keys for details.