Inkscape

Going back to the Flood effect's settings there are two options to alter the output.
A slider can set the opacity of the colour between fully transparent and opaque, representing it with a value between 0 and 1.

Clicking on the Flood Color it brings up a color picker panel, with multiple inputs.
Hex color names and RGB values are of the same as in the fill and stroke panel, however the HSL wheel is not 0-255 based: hue has a 0-360 scale, while the saturation and lightness can be set between 0-100 values.

An important feature is that there is also a dropper tool included with the panel which can take any part of the screen as a reference.

Let's change the default black to red.