Color Mix(Color colorA, Color colorB, double fracA)
{
    byte r = (byte)((colorA.R * (1 - fracA)) + colorB.R * fracA);
    byte g = (byte)((colorA.G * (1 - fracA)) + colorB.G * fracA);
    byte b = (byte)((colorA.B * (1 - fracA)) + colorB.B * fracA);
    return Color.FromArgb(r, g, b);
}

Color Mix(string hexA, string hexB, double fracA)
{
    Color colorA = ColorTranslator.FromHtml(hexA);
    Color colorB = ColorTranslator.FromHtml(hexB);
    return Mix(colorA, colorB, fracA);
}

for (int i = abf.sweepCount - 1; i >= 0; i--)
{
    double frac = (double)i / (abf.sweepCount - 1);
    Color color = Mix("#8181f2", "#a2f0ca", frac);
    var trace = abf.GetSweep(i);
    plt.PlotSignal(
        ys: trace.values,
        sampleRate: trace.sampleRate,
        color: color,
        yOffset: i * 15,
        xOffset: i * .025, 
        maxRenderIndex: 20000, lineWidth: 2);
}