我想这是因为我阅读的是3字节（RGB）而不是4字节（ARGB）
是的，这就是问题的关键。如果你想处理原始图像内容，你必须依赖PixelFormat。你必须区分索引格式（8bpp或更少），BitmapData中的像素不是颜色，而是调色板的索引。

public void ChangeColor(Bitmap bitmap, Color from, Color to)
{
    if (Image.GetPixelFormatSize(bitmap.PixelFormat) > 8)
    {
        ChangeColorHiColoredBitmap(bitmap, from, to);
        return;
    }

    int indexFrom = Array.IndexOf(bitmap.Palette.Entries, from);
    if (indexFrom < 0)
        return; // nothing to change

    // we could replace the color in the palette but we want to see an example for manipulating the pixels
    int indexTo = Array.IndexOf(bitmap.Palette.Entries, to);
    if (indexTo < 0)
        return; // destination color not found - you can search for the nearest color if you want

    ChangeColorIndexedBitmap(bitmap, indexFrom, indexTo);
}

private unsafe void ChangeColorHiColoredBitmap(Bitmap bitmap, Color from, Color to)
{
    int rawFrom = from.ToArgb();
    int rawTo = to.ToArgb();

    BitmapData data = bitmap.LockBits(new Rectangle(Point.Empty, bitmap.Size), ImageLockMode.ReadWrite, bitmap.PixelFormat);
    byte* line = (byte*)data.Scan0;
    for (int y = 0; y < data.Height; y++)
    {
        for (int x = 0; x < data.Width; x++)
        {
            switch (data.PixelFormat)
            {
                case PixelFormat.Format24bppRgb:
                    byte* pos = line + x * 3;
                    int c24 = Color.FromArgb(pos[0], pos[1], pos[2]).ToArgb();
                    if (c24 == rawFrom)
                    {
                        pos[0] = (byte)(rawTo & 0xFF);
                        pos[1] = (byte)((rawTo >> 8) & 0xFF);
                        pos[2] = (byte)((rawTo >> 16) & 0xFF);
                    }
                    break;
                case PixelFormat.Format32bppRgb:
                case PixelFormat.Format32bppArgb:
                    int c32 = *((int*)line + x);
                    if (c32 == rawFrom)
                        *((int*)line + x) = rawTo;
                    break;
                default:
                    throw new NotSupportedException(); // of course, you can do the same for other pixelformats, too
            }
        }

        line += data.Stride;
    }

    bitmap.UnlockBits(data);
}

private unsafe void ChangeColorIndexedBitmap(Bitmap bitmap, int from, int to)
{
    int bpp = Image.GetPixelFormatSize(bitmap.PixelFormat);
    if (from < 0 || to < 0 || from >= (1 << bpp) || to >= (1 << bpp))
        throw new ArgumentOutOfRangeException();

    if (from == to)
        return;

    BitmapData data = bitmap.LockBits(
        new Rectangle(Point.Empty, bitmap.Size),
        ImageLockMode.ReadWrite,
        bitmap.PixelFormat);

    byte* line = (byte*)data.Scan0;

    // scanning through the lines
    for (int y = 0; y < data.Height; y++)
    {
        // scanning through the pixels within the line
        for (int x = 0; x < data.Width; x++)
        {
            switch (bpp)
            {
                case 8:
                    if (line[x] == from)
                        line[x] = (byte)to;
                    break;
                case 4:
                    // First pixel is the high nibble. From and To indices are 0..16
                    byte nibbles = line[x / 2];
                    if ((x & 1) == 0 ? nibbles >> 4 == from : (nibbles & 0x0F) == from)
                    {
                        if ((x & 1) == 0)
                        {
                            nibbles &= 0x0F;
                            nibbles |= (byte)(to << 4);
                        }
                        else
                        {
                            nibbles &= 0xF0;
                            nibbles |= (byte)to;
                        }

                        line[x / 2] = nibbles;
                    }
                    break;
                case 1:
                    // First pixel is MSB. From and To are 0 or 1.
                    int pos = x / 8;
                    byte mask = (byte)(128 >> (x & 7));
                    if (to == 0)
                        line[pos] &= (byte)~mask;
                    else
                        line[pos] |= mask;
                    break;
            }
        }

        line += data.Stride;
    }

    bitmap.UnlockBits(data);
}

您发布的代码中有三个不同的问题：
1.颜色组件顺序错误。Bitmap类以little-endian格式将像素值存储为整数。这意味着组件的字节顺序实际上是BGR（或32 bpp的BGRA）。
1.在VB .NET中，你不能直接比较Color值。我对VB .NET的了解不够，不知道为什么会这样，但我认为这是与VB .NET如何处理值类型相关的正常语言行为。要正确比较Color值，你需要调用ToArgb()，它返回一个Integer值，可以直接比较。
1.您的For循环使用了错误的结束值。如果仅从数组长度中减去1，则循环可能会遇到行尾的填充，但发现字节太少，无法成功地将2添加到循环索引并仍保留在数组中。
下面是一个对我来说很好用的扩展方法：

<Extension>
Public Function ChangeColor(ByVal image As Image, ByVal oldColor As Color, ByVal newColor As Color)
    Dim newImage As Bitmap = New Bitmap(image.Width, image.Height, image.PixelFormat)

    Using g As Graphics = Graphics.FromImage(newImage)
        g.DrawImage(image, Point.Empty)
    End Using

    ' Lock the bitmap's bits.
    Dim rect As New Rectangle(0, 0, newImage.Width, newImage.Height)
    Dim bmpData As BitmapData = newImage.LockBits(rect, ImageLockMode.ReadWrite, newImage.PixelFormat)

    ' Get the address of the first line.
    Dim ptr As IntPtr = bmpData.Scan0

    ' Declare an array to hold the bytes of the bitmap. 
    Dim numBytes As Integer = (bmpData.Stride * newImage.Height)
    Dim rgbValues As Byte() = New Byte(numBytes - 1) {}

    ' Copy the RGB values into the array.
    Marshal.Copy(ptr, rgbValues, 0, numBytes)

    ' Manipulate the bitmap.
    For i As Integer = 0 To rgbValues.Length - 3 Step 3

        Dim testColor As Color = Color.FromArgb(rgbValues(i + 2), rgbValues(i + 1), rgbValues(i))

        If (testColor.ToArgb() = oldColor.ToArgb()) Then
            rgbValues(i) = newColor.B
            rgbValues(i + 1) = newColor.G
            rgbValues(i + 2) = newColor.R
        End If

    Next i

    ' Copy the RGB values back to the bitmap.
    Marshal.Copy(rgbValues, 0, ptr, numBytes)

    ' Unlock the bits.
    newImage.UnlockBits(bmpData)

    Return newImage

End Function

就这一点而言：
我不知道如何适应任何源像素格式，我可以传递给函数。
不幸的是，API不会直接返回位图的每像素位数或每像素字节数，您可以泛化代码以考虑每像素字节数，但仍必须至少将PixelFormat值Map到每像素字节数。