Easy to build and fun to marvel at, get ready to make a Lego Medical Sign!
123 of each color for three colors.
Start by building a structure 3 layers high and that's staggered on the end as shown in the first and second pictures. The top level should be 9 studs (those dots on top the bricks) long. Turn the 90 degree corner and continue the strip until it's 14 studs long. Now you should have a backwards L. Continue attaching bricks so that you build another line of 14 studs going right next to the last.
Notice that the first back to back Ls were topped off by two vertical bricks but the next L pair is topped off by a horizontal brick. As you build, these will continue to go back and forth.