Gradients are used to visualize fluid particle channels like velocity or vorticity. You find them with domain and emitter nodes:

A click on the “+” icon opens the gradient tools:

Colour knots can be moved freely – their absolute positions are displayed in percent under “Position”. 0 = outer left position, 100 = outer right position.
When active, a colour knot's RGB components are editable. A colour can be selected with a click on the colour filed or adjusted with the RBG/HSV sliders.
New colour knots are added with a click on the grey bar below the gradient. A new bias knot is also created.
To remove a colour knot, right-click on it, and choose “Remove color knot”. The corresponding bias knot will be removed as well.

Bias knots can be shifted to control a colour's influence.
New bias knots are created automatically when a new marker is added.
When a bias knot is active its position is measured between its corresponding colour knots. Under “Position” you will see a percent value ranging from 0 (= left colour knot) to 100 (= right colour knot).

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