WCAG 2 level AA requires a contrast ratio of at least 4.5:1 for normal text and 3:1 for large text, and a contrast ratio of at least 3:1 for graphics and user interface components (such as form input borders). Level AAA requires a contrast ratio of at least 7:1 for normal text and 4.5:1 for large text. Large text is defined either as bold and at least 14 point (typically 18.66px) or regular and at least 18 point (typically 24px).
Large text is defined as 14 point (typically 18.66px) and bold or larger, or 18 point (typically 24px) or larger.
Gradually mixing one color with another to create pleasing gradients. Changing the color space changes the way those colors are mixed together and will greatly affect the result.
CIELAB was designed to be perceptually uniform with respect to human color vision, meaning that the same amount of numerical change in these values corresponds to about the same amount of visually perceived change. This is especially visible when the gradient space is set to lightness. Read more
Is a mix of red green and blue as natively produced by the screen you are reading this on. By choosing the gradient space ‘red’, the color red will gradually be mixed to the amount of ‘blue’ and ‘green’ that you have chosen on the corresponding sliders.
Those colors are generated by a helix (spiral) traversing the RGB color space. Looking at the 3-D view should explain this visually pretty nicely. Cubehelix was designed to provide a color mapping that would degrade gracefully to grayscale without losing information. Also meaning that there is a clear hierarchy in lightness. Read more
HSL stands for Hue Saturation and light. The palette is generated by rotating on the hue parameter.
HULuv & HPLuv
The input in HULuv and HPLuv is the same as in HSL. The generated colors are corrected for perceptual uniformity using the findings from CIE. On top of that HPLuv aims to correct the chroma, meaning that not all the colors from the RGB space can be represented. Read more