combinatorics- coloring of points in plane

There are k colors given. Every point in a 2 D plane carries  exactly one of these k colors. what are the values of k, for which there will necessarily exist a rectangle with vertices having same color. In particular, what is maximum value of k. 

coin weighing

There are   10 piles containing large number of coins. Coins in same pile all weigh same but may differ from coins in other piles. It is given that all  coins weigh 1 gram, 2 gram or 3 gram. You have a digital weighing machine, to measure any weight. You can pick as many coins as you wish from the piles.How will you ascertain weight of all coins in minimum number of weighments.