The yellow portion is rice coloured with mashed egg yolk and a bit of tumeric.
The bento is also packed with miso chicken, broccoli and strawberries.
I've some request for a tutorial to do this. I was a bit hesistant to share as I'm not doing it the correct way. But on second thoughts, I decided I'll do the tutorial so that if you are like me and can't roll sushi the proper way, you might want to give this method a try. :)
Scoop out a portion of rice and mixed with mashed up egg yolk. If you find the colour not yellow enough, you can add in a bit of tumeric or one more egg yolk.
Using cling wrap, compress and shape the yellow rice with your hands into a tube like the picture above. You can use a sheet of seaweed as a guide, the length of the rice should be the around the same as the length of the seaweed. The cross-section of this should be oval. Cut out a suitable amount of seaweed and wrap it around the rice.
Using the same method as above, shape 2 smaller tubes of white rice. Wrap with seaweed after you are done.
You can use a sushi mat for this step. I just used my hand to roll. :P Lay a sheet of nori on a sushi mat, spread a thin layer of rice on a sheet of nori, put the yellow tube of rice at the bottom.
The 2 white tubes of rice should go on top of the yellow one. Using the sushi mat, roll and seal up the sushi roll.
Wet your knife before you slice the sushi. It should look roughly like this after cutting.
The puncher I used to get the details. The stripes are cut with a pair of scissors. I used decor gel for the cheeks.
There, it's done. Sorry, I'm not good at explaining but it's not difficult to do, hopefully the picture will help you to visualise the steps better. :)