Bulma has custom Sass functions to help find related colors dynamically:

findColorInvert($color): returns either 70% transparent black or 100% opaque white depending on the luminance of the color
findLightColor($color): returns the current color but with a lightness of at least 96%
findDarkColor($color): returns the current color but with a lightness of at most 29%

Bulma also has a few utility functions to calculate useful values:

powerNumber($number, $exp): calculates the value of a number exposed to another one. Returns a number
colorLuminance($color): defines if a color is dark or light. Return a decimal number between 0 and 1 where <= 0.5 is dark and > 0.5 is light

The findColorInvert($color) function takes a color as an input, and outputs either transparent black rgba(#000, 0.7) or white #fff:

if colorLuminance($color) > 0.55, it outputs rgba(#000, 0.7)
otherwise, it outputs #fff

Its purpose is to guarantee a readable shade for the text when the input color is used as the background.

color	color luminance	findColorInvert()	result
`#00d1b2`	`0.52831`	`#fff`	Button
`#3273dc`	`0.23119`	`#fff`	Button
`#23d160`	`0.51067`	`#fff`	Button
`#ffdd57`	`0.76863`	`rgba(0, 0, 0, 0.7)`	Button
`#ff3860`	`0.27313`	`#fff`	Button
`#ffb3b3`	`0.61796`	`rgba(0,0,0,0.7)`	Button
`#ffbc6b`	`0.63053`	`rgba(0,0,0,0.7)`	Button
`hsl(294, 71%, 79%)`	`0.5529`	`rgba(0,0,0,0.7)`	Button

For colors that have a luminance close to the 0.55 threshold, it can be useful to override the findColorInvert() function, and rather set the invert color manually.
For example, this shade of purple has a color luminance of 0.5529. It can be preferable to set a color invert of white instead of transparent black:

with `findColorInvert()`	`$purple-invert: findColorInvert($purple)`	`rgba(0,0,0,0.7)`	Button
with manual setting	`$purple-invert: #fff`	`#fff`	Button

The findLightColor($color) and findDarkColor($color) functions take a color as an input, and output that color's light and dark versions respectively: