你可以通过自定义适配器来解决Android GridView计算背景颜色不均匀的问题。以下是一个示例代码：

首先，创建一个名为MyAdapter的自定义适配器类：

public class MyAdapter extends BaseAdapter {
    private Context mContext;
    private List mColors;

    public MyAdapter(Context context, List colors) {
        mContext = context;
        mColors = colors;
    }

    @Override
    public int getCount() {
        return mColors.size();
    }

    @Override
    public Object getItem(int position) {
        return mColors.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;

        if (convertView == null) {
            LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.grid_item_layout, null);

            holder = new ViewHolder();
            holder.colorView = (ImageView) convertView.findViewById(R.id.color_view);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        int color = mColors.get(position);
        holder.colorView.setBackgroundColor(color);

        return convertView;
    }

    static class ViewHolder {
        ImageView colorView;
    }
}

然后，创建一个名为grid_item_layout.xml的布局文件，用于定义每个GridView的项的布局：

最后，将适配器应用到GridView上：

GridView gridView = findViewById(R.id.grid_view);
List colors = Arrays.asList(Color.RED, Color.GREEN, Color.BLUE, Color.YELLOW, Color.MAGENTA, Color.CYAN);
MyAdapter adapter = new MyAdapter(this, colors);
gridView.setAdapter(adapter);

以上代码中，colors列表包含了要显示的颜色，MyAdapter将颜色应用到每个GridView项的背景上。这样，GridView的背景颜色就会均匀显示了。