Make a small dot in the top center of one sheet of cardstock to serve as a starting point for the apex of the pyramid. Position the inside edge of your ruler at a 45 degree angle, stretching from the lower left of the paper toward the upper right, across the dot. Draw a straight line along the edge of the ruler. Press down on the top of the ruler---the portion that touches the dot---and slide the ruler across your paper so that it angles 45 degrees, from the top left to the bottom right. Draw another line of the same length along the ruler's edge.
Remove the ruler and look at your two lines; they are the pyramid's right and left sides. Slide the ruler horizontally across the bottoms of the two lines and draw a third line to create finish a perfectly symmetrical triangle.
Color the inside of the pyramid however you wish, using ink, paint or pencil. If you want to have colored blocks, draw horizontal and vertical lines across the front of the pyramid to create blocks that you can color.
Cut the triangle out using scissors and use the loose triangle to trace additional shapes onto the other sheets of cardstock. Cut them out and color them as you wish. Stand one triangle on its base and place another next to it with the right edge of one flush against the left edge of the other. Tape the edges together. Stand the other two remaining triangles together and tape them together. Tape the paired triangles together to form your colored 3D pyramid.