Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 81

Figure 2.52. Set Key holds the current value at the current frame in place.
This creates a keyframe for the four values (R, G, B, and Alpha) associated with
the Gain property. Notice that the field turns a blueish color—this is to show that
a keyframe is present on this frame for this property ( FIGURE 2.53 ).
Figure 2.53. The blue field indicates this frame has a keyframe.
The Timebar also displays a blueish-colored little box to indicate on which frame
you have keyframes ( FIGURE 2.54 ) . The markings on the Timebar are shown
for all open property panels that have keyframes. To see the keyframes on the
Timebar for a specific node, make sure only that node's Properties panel is open
in the Properties Bin.
Figure 2.54. The blue boxes on the Timebar indicate keyframes.