My kid has been using the red and blue one for producing oo and oh sound and worked so far :)
I bought this from an intervention center and we have a OPT therapist right now but we are not using the yellow tube yet. For what sound is the yellow one?

The shapes and tubes are wonderful tactile cues for my tiny ones with Apraxia or Suspected Apraxia to elicit bilabials and /u/ vowel. My kids have learned to associate the sounds with the shapes and just the visual of the shape and tactile cue on the lips is enough to elicit the sound/or approximation!

The Apraxia tubes are a “go to” for children who need extra sensory input when forming lip rounding for /oo/ (boo!) and /oh/ (go!) sounds. It’s hard for children with low muscle tone and strength to pucker. These have helped many children on my caseload to achieve these sounds beautifully! I usuallly use them in conjunction with the bite tube hierarchy, straw and horn hierarchies. They are awesome! Thanks Renee for the Apraxia course too! It helped me to understand how to use them!

These shapes are great tactile cues for developing speech in the birth-3 and pre-school population. I've had great success using these with not only children with tone and sensory issues, but also typically developing children with speech delays.