The slow point mainly refers to allocating and copy into a new buffer. The naive points refers to that you use 1) alloca to allocate an unknown amount of memory on the stack and 2) assume that write is atomic, never fails and will write the whole buffer (you return count). Instead you should return what write actually wrote or not. There are also a whole can of worms with regards to what happens during (signal) interrupt or if write failed to write the whole buffer. In which case the reset color code may not be sent etc etc. Solutions to this require some fine-thinking, but for the slow stuff, you can simply replace all the allocation and copy with something like this

  struct iovec iov[3] = {{STDERR_COLOR, STDERR_COLOR_SIZE},{buf, count},{COL_RESET, COL_RESET_SIZE}};
  ssize_t n = 0;
  do { n = writev(2, iov, 3);} while (n == -1 && errno == EINTR);
  return n;