Compressing Texture2D with SetPixel() return weird colors in Unity

Question

As the title said, I'm compressing an image (inputed as a Texture2D) by dividing the image into blocks and taking the average r, g, and b value.

Image:

But the result is mostly gray-ish color with a diagonal strip of other colors in the middle:

This is the code for compression:

private void ASCIIConvert()
{
    texWidth = tex.width; texHeight = tex.height;

    int blockHeight = Mathf.FloorToInt((float)texHeight / neededHeight);
    int blockWidth = Mathf.FloorToInt((float)texWidth / neededWidth);
    int maxHeight = blockHeight * neededHeight;
    int maxWidth = blockWidth * neededWidth;
    int numBlockHeight = maxHeight/blockHeight;
    int numBlockWidth = maxWidth/blockWidth;

    tex2 = new Texture2D(numBlockWidth, numBlockHeight);
    for (int idx = 0; idx < maxHeight; idx += blockHeight)
    {
        int curBlockHeight = Mathf.FloorToInt((float) idx / blockHeight);

        for (int j = 0; j < maxWidth; j += blockWidth)
        {
            float sumr = 0, sumg = 0, sumb = 0;
            int curBlockWidth = Mathf.FloorToInt((float) idx / blockWidth);

            for (int m = idx * blockHeight; m < idx * blockHeight + blockHeight; m++)
            {
                for (int n = j * blockWidth; n < j * blockWidth + blockWidth; n++)
                {
                    sumr += tex.GetPixel(n,m).r;
                    sumg += tex.GetPixel(n,m).g;
                    sumb += tex.GetPixel(n,m).b;
                }
            }

            sumr /= (blockWidth * blockHeight);
            sumg /= (blockWidth * blockHeight);
            sumb /= (blockWidth * blockHeight);

            tex2.SetPixel(curBlockWidth, curBlockHeight, new Color(sumr, sumg, sumb, 1f));
        }
    }
    tex2.Apply();

    //standard
    newSprite = Sprite.Create(tex2, new Rect(0.0f, 0.0f, tex2.width, tex2.height), new Vector2(0.5f, 0.5f), 100.0f);
    imgContainer.GetComponent<SpriteRenderer>().sprite = newSprite;
}

neededWidth and neededHeight are length and width of the wanted compressed texture. Any idea where the compression may have gone wrong?