Open any CSS file and you'll find colors written as #1a73e8, rgb(26, 115, 232), or hsl(217, 89%, 51%). They can all describe the exact same shade of blue -- the difference isn't precision, it's what each format makes easy to do.
HEX: compact and copy-paste friendly
HEX packs red, green, and blue into a six-digit code, two digits per channel, each running from 00 to ff (0-255 in hexadecimal). It's the format you'll see most in design tools and style guides mainly because it's short and unambiguous to paste around. The catch is that HEX is opaque to read -- #1a73e8 tells you nothing about how light, dark, or saturated the color is until you decode it.
RGB: the format that matches how screens work
RGB spells out the same three channels in plain decimal: rgb(26, 115, 232). It's more verbose than HEX but easier to reason about if you're used to thinking in 0-255 ranges, and it's the natural format when you're generating colors programmatically -- averaging two colors, adjusting one channel, or reading pixel data from a canvas all work directly in RGB math. Add a fourth value and you get RGBA, where the last number is opacity from 0 to 1.
HSL: the one built for adjusting a color, not just storing it
HSL describes color differently: hue (the position on a color wheel, 0-360 degrees), saturation (how vivid vs. gray, 0-100%), and lightness (how close to black or white, 0-100%). This is the format that actually matches how people think about adjusting color. Want a darker version of the same blue? Lower the lightness and leave hue and saturation untouched. Want a muted version? Drop the saturation. Try doing either of those with HEX or RGB and you're recalculating three interdependent numbers instead of changing one.
This is also why HSL shows up constantly in generated color palettes and theme systems -- picking a base hue and then varying lightness across a handful of steps is exactly how most "shades of a color" UI components are built.
CMYK is the odd one out
If you've ever prepared artwork for print, you've likely run into CMYK -- cyan, magenta, yellow, and black. It exists because printers mix physical ink rather than emitting light, so it's a fundamentally different model (subtractive instead of additive). A color that looks right in RGB on screen can shift when converted to CMYK for print, which is why print shops ask for CMYK values specifically rather than trusting a straight RGB-to-CMYK conversion done blind.
Picking one in practice
There's no format you're locked into -- browsers accept HEX, RGB(A), and HSL(A) interchangeably in CSS, so the choice comes down to what you're doing with the value. Storing a brand color or copying a design spec: HEX. Computing or animating a color in code: RGB. Building a palette or tweaking a shade: HSL. Sending a file to a print vendor: CMYK.
If you've got a color in one format and need it in another, the Color Converter handles HEX, RGB, HSL, and CMYK conversions in one place, so you don't have to do the math by hand.