Free color tools
Every checker on the web tells you a pair fails and stops there. These give you the color to use instead, worked out in OKLCh with your hue held exactly. They run entirely in your browser, take no account, and never look at your website.
Contrast checker
Two colors, the ratio, and the nearest color that passes.
Palette matrix
Every pair in your palette, checked at once.
Accessible ramp
One brand color, a full scale, every step measured.
Color blindness check
Two colors that pass contrast can still be the same color.
Focus and UI check
Fifteen interface parts, and which of them the 3:1 rule exempts.
These work on colors you already have
Which is the easy half. Knowing which pairs are actually on your page, after the cascade, after JavaScript, after the semi-transparent panel, is the half that needs a real browser. That is what the scan does, in light and dark separately, and it is free to run.