expando_callback_color

static void simple_s(const struct ExpandoNode *node, void *data,
                     MuttFormatFlags flags, struct Buffer *buf)
{
  assert(node->type == ENT_EXPANDO);

  const struct SimpleExpandoData *sd = data;
  struct ExpandoExpandoPrivate *p = node->ndata;

  p->color = MT_COLOR_INDEX_SUBJECT;

  const char *s = NONULL(sd->s);
  buf_strcpy(buf, s);
}

static void simple_C(const struct ExpandoNode *node, void *data,
                     MuttFormatFlags flags, struct Buffer *buf)
{
  assert(node->type == ENT_EXPANDO);

  const struct SimpleExpandoData *sd = data;
  struct ExpandoExpandoPrivate *p = node->ndata;

  p->color = MT_COLOR_INDEX_NUMBER;

  const int num = sd->C;
  buf_printf(buf, "%d", num);
}