How HEX and RGB relate
A HEX color such as #1d4ed8 is three pairs of hexadecimal digits for red, green and blue. 1d is 29, 4e is 78 and d8 is 216, so the RGB value is rgb(29, 78, 216). Three-digit shorthand like #abc expands to #aabbcc.
HSL
HSL describes the same color as hue (0 to 360 degrees on the color wheel), saturation and lightness. It is convenient for making lighter or darker variants: change only the lightness.
Accessibility tip
Text needs enough contrast against its background. WCAG recommends a contrast ratio of at least 4.5:1 for normal text.