Enter a color in any common format and get it back in all the others, with a live swatch. The contrast checker scores the color against white and black text to WCAG standards, and the shade generator gives you a tint and shade ramp to build a palette from.
RGB mixes red, green and blue light, which is how screens work but is unintuitive to adjust by hand. HSL describes hue, saturation and lightness, so making a color lighter or less saturated is a single number change. They describe the same colors.
WCAG AA requires 4.5:1 for normal text and 3:1 for large text. AAA requires 7:1 and 4.5:1. The checker shows the ratio against white and black and which levels pass.