I found a example of what I want here Chart JS plugin to change line color depending on value
but I'm not able to make it work right. Trying to simply it down to figure it out. Playing with this.
const gradientFill = c.ctx.createLinearGradient( x_start, y_start, x_end, y_end);
gradientFill.addColorStop(0, "red");
gradientFill.addColorStop(1, "blue");
const model = x.data.datasets[0]._meta[Object.keys(dataset._meta)[0]].dataset._model;
model.borderColor = gradientFill;
this works but I do not want the gradient color, just a two tone red or blue.