I think you could immediately shorten that to:

// This if statement might seem kind of magical
if (((rects.size()  / 8) % 2) == 0) // if <= 16, 32, 48, 64
{
    if(j%2 == 1  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 0)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}
else // <= 8, 24, 40, 56
{
    if(j%2 == 0  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 1)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}

However, I wouldn't actually recommend the above, and if I were you, I'd do something more like:

for (int i = 0; i < rects.size(); ++i)
{
    if (((i + i / 8) % 2) == 0)
    {
        rects[i]->set_fill_color(Color::red);
    }
    else
    {
        rects[i]->set_fill_color(Color::white);
    }
}

Edited: sorry, I messed up the above the first time. I think it's good now...

Edit again: Another alternative is the below double loop:

bool white = false;
for (int i = 0; i < rects.size(); i += 8)
{
    for (int k = 0; k < 8; ++k)
    {
        if (white)
        {
            rects[i + k]->set_fill_color(Color::white);
        }
        else
        {
            rects[i + k]->set_fill_color(Color::red);
        }

        white = !white;
    }

    white = !white;
}

I think you could immediately shorten that to:

// This if statement might seem kind of magical
if (((rects.size()  / 8) % 2) == 0) // if <= 16, 32, 48, 64
{
    if(j%2 == 1  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 0)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}
else // <= 8, 24, 40, 56
{
    if(j%2 == 0  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 1)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}

However, I wouldn't actually recommend the above, and if I were you, I'd do something more like:

for (int i = 0; i < rects.size(); ++i)
{
    if (((i + i / 8) % 2) == 0)
    {
        rects[i]->set_fill_color(Color::red);
    }
    else
    {
        rects[i]->set_fill_color(Color::white);
    }
}

Edited: sorry, I messed up the above the first time. I think it's good now...

Edit again: Another alternative is the below double loop:

bool white = false;
for (int i = 0; i < rects.size(); i += 8)
{
    for (int k = 0; k < 8; ++k)
    {
        if (white)
        {
            rects[i]->set_fill_color(Color::white);
        }
        else
        {
            rects[i]->set_fill_color(Color::red);
        }

        white = !white;
    }

    white = !white;
}

I think you could immediately shorten that to:

// This if statement might seem kind of magical
if (((rects.size()  / 8) % 2) == 0) // if <= 16, 32, 48, 64
{
    if(j%2 == 1  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 0)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}
else // <= 8, 24, 40, 56
{
    if(j%2 == 0  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 1)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}

However, I wouldn't actually recommend the above, and if I were you, I'd do something more like:

for (int i = 0; i < rects.size(); ++i)
{
    if (((i + i / 8) % 2) == 0)
    {
        rects[i]->set_fill_color(Color::red);
    }
    else
    {
        rects[i]->set_fill_color(Color::white);
    }
}

Edited: sorry, I messed up the above the first time. I think it's good now...

I think you could immediately shorten that to:

// This if statement might seem kind of magical
if (((rects.size()  / 8) % 2) == 0) // if <= 16, 32, 48, 64
{
    if(j%2 == 1  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 0)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}
else // <= 8, 24, 40, 56
{
    if(j%2 == 0  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 1)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}

However, I wouldn't actually recommend the above, and if I were you, I'd do something more like:

for (int i = 0; i < rects.size(); ++i)
{
    if ((i % 2) == 0)
    {
        rects[i]->set_fill_color(Color::red);
    }
    else
    {
        rects[i]->set_fill_color(Color::white);
    }
}

It would help if I knew what rects, j, and k were, but (I think) you could immediately shorten that to:

// This if statement might seem kind of magical
if (((rects.size()  / 8) % 2) == 0) // if <= 16, 32, 48, 64
{
    if(j%2 == 1  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 0)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}
else // <= 8, 24, 40, 56
{
    if(j%2 == 0  )
    {
        (*rects[(k)]).set_fill_color(Color::red);
    }
    else if(j%2 == 1)
    {
        (*rects[(k)]).set_fill_color(Color::white);
    }
}

However, I wouldn't actually recommend the above, and if I were you, I'd do something more like:

for (int i = 0; i < rects.size(); ++i)
{
    if ((i % 2) == 0)
    {
        rects[i]->set_fill_color(Color::red);
    }
    else
    {
        rects[i]->set_fill_color(Color::white);
    }
}