It's a simple matter to apply a color fill to an object. Simply select the fill tool and then just click and drag across the object you want to fill - in this case a simple magenta colored rectangle.

No need for opening menus and dialogs or specifying values - a fill is applied immediately on releasing the mouse, following the angle and depth of the fill arrow. By default this is a linear fill from white to the starting color of the object.

[Note you can change to a truly interactive fill tool by pressing TAB as you drag. Try it and see - the fill arrow doesn't show until you release the mouse, but the fill follows the position of the mouse as you move it.]

The angle and length of the fill can changed by dragging the start or end of the arrow to a new position. Note: this can be completely outside the object to achieve a more subtle graduation.

Changing colors in the fill is equally easy. Simply drag and drop new colors from the color line at the bottom of the document onto the fill arrow - a new fill handle [the square blob marking the position of the color] appears for each color. You can simply drag these fill handles along the arrow to reposition the colors in the fill and delete them to remove a color from the fill.

You can change the type of fill:

...to circular, where the color radiates out from the center point...

...to elliptical, where the color radiates out in a similar way to the circular but with 2 fill handles to stretch the color in 2 different directions...

...to conical, where the color sweeps around from a central point

...to diamond, where the colors graduate out from the center point in a diamond shape

...to 3 and 4 color fills which give you 3 and 4 color fill handles - best to think of this like the effect of having 3 or 4 color spotlights shining on the object [I've added dark blue and then yellow].

...to fractal clouds or fractal plasma which produce a cloud-like effect.

You can change the way that colors tile across a fill in the Tiling drop down menu on the fill infobar. 'Simple' means there is a single copy of the fill and 'repeating' tiles multiple copies along the length of the fill [except flat and conical fills where tiling isn't possible.]

You can also alter the way that the colors change from one to another in a fill in the Fill Effect drop down menu on the fill infobar. 'Fade' means they simply fade from one to another but you can also get some very colorful effects by using rainbow or alt rainbow. This is an Alt Rainbow fill - the colors transition from magenta to cyan around the edge of the HSV color wheel.

The color fill also has a profile control, which you can use if you want a non-linear rate of change in the color of the fill (not suitable for flat or 3 or 4 color fills). Click on the arrow on the color tool infobar...

..and experiment by dragging the sliders, which alter the rate of change of the color across the fill. In this example [a linear fill from cyan to magenta] the colors change slowly at first and then quickly at the end of the fill.