Show HN: /r/place backed by the Bitcoin blockchain

IUUC the current code accumulate the payment to each pixel and whatever color has more accumulated money is chosen.

IIUC your code only consider the last payment and to replace the old color you have to outbid it, without any consideration of the previous payments.

Another strategies:

Penny auction: Whatever color received the last payment is the chosen one. This is more similar to /r/place

Generals.io: If someone adds money to the current color, then increase the accumulated money in that color. If someone adds money to another color the amount is discounted. Unless the result is negative and the color changes.

This code suffers from Recursive calling vulnerability (same bug which caused the DAO to be hacked).

You're sending the money BEFORE you're updating the balances.

There is a better way to write the payment code (and also wouldn't take you more than 5 minutes to write).

Here is one way to do it:

    function colorPixel(uint x, uint y, bytes3 color) payable onBoard(x, y) { 
      Pixel p = board[x][y];
      lastPayment = p.lastPayment;
      p.lastPayment = msg.value;
      if (msg.value > lastPayment) {
        if(!msg.sender.send(lastPayment)) throw; // refund previous price of pixel
        p.color = color;                // set the color
      } else {
        throw;
      }
    }

    function colorPixel(uint x, uint y, bytes3 color) payable onBoard(x, y) { 
      Pixel p = board[x][y];
      require(msg.value > p.lastPayment);
      if(!msg.sender.send(p.lastPayment)) throw; // refund previous price of pixel
      p.color = color;                // set the color
      p.lastPayment = msg.value;      // set new cost to whatever the person sent in
    }

That said, cryptocurrency is much more niche than, say, an open-access canvas requiring only a Reddit account.

This is not as dynamic as Reddit Place, the code is waiting for 1 confirmation before the pixel is colored.