I used the Flashback Skinny Tee pattern from Made by Rae in 12-18 months for my chunky 9 month old. (LOVE this pattern!) You will need 3 different color knits but make sure that they are all the same type/weight of knit.
Once you decide on the orientation of your colors use the diagrams above to cut out your pieces, 3 for front and 3 for back. Your pieces should look similar to the 3 above. Take piece 1 and piece 2 and place right sides together and sew with a 3/8 inch seam. I used my serger but since it's knit you could as easily use a regular sewing machine and it won't fray. Just be sure to use a stitch with some stretch as the shirt will need to stretch over the belly.
Now place right sides of piece 2 and piece 3 together and seam. Press seams down towards the bottom of the shirt. Repeat with other three color block pieces.
Fold each color block piece in half making sure the seams of each color line up. Place your pattern on the fold and cut. You should piece should be similar to this:
Cut out your sleeve pieces and put the shirt together per pattern instructions.
ta-da! Your very own teeny color block tee.
first version here!