test.c

typedef struct _object {
    _PyObject_HEAD_EXTRA
    Py_ssize_t ob_refcnt;
    int color;        /* the color of local mark-scan where 0 black, 1 red 2 green */
    struct _typeobject *ob_type;
} PyObject;


#define _Py_CHECK_REFCNT(OP)                                    \
{       if (((PyObject*)OP)->ob_refcnt < 0)                             \
                _Py_NegativeRefcount(__FILE__, __LINE__,        \
                                     (PyObject *)(OP));         \
        else
                
}

You can assign the color just writing:

((PyObject*)OP)->color = 0