how to defile line in specific length in the LED strip which is filled

how to defile line in specific length in the LED strip which is filled color (solid color, rainbow,spark , flame and any pallet ) & background color & speed could be customized as function argument

any help really appreciated

I haven’t defiled anything lately (the week is still young), however some of my _demo examples at https://github.com/atuline/FastLED-Demos allow for variable speed, colour, etc.