Colour Replacer
Click a colour in the picture, choose what it should become, and set how close a colour has to be to count. Replace it with another colour or with nothing at all, and save a PNG. Your file never leaves this tab.
Reading happens on your machine. Nothing is being sent anywhere.
A colour just outside the tolerance fades into the swap instead of stopping dead, which keeps anti-aliased edges smooth.
On, the swap moves every matching pixel by the same step, so shadows and highlights survive. Off, every match becomes exactly the new colour, like a paint bucket.
Matching is done in CIE L*a*b*, where the distance between two colours matches how different they look, so the closeness slider follows your eye rather than the RGB numbers: about 2 is the point where most people stop seeing any difference at all, 10 covers a shade, 25 covers the lighter and darker versions of a colour, and 60 covers a whole family. Transparency is written into the alpha channel, so save as PNG if you use it; a JPEG has no alpha to write it into.