This can be done using a Canvas and a BMI table with 8 BMI ranges and their colors.
(I say 8 because I see 8 color ranges in your picture. )
See Chapters 2 and 17 of the free book App Inventor 2 Book: Create Your Own Android Apps
from FAQ Section: Books, Tips, Tutorials for AI2
to learn about the Canvas.
You will need a little math to turn the BMI (29.4) lowest and highest values range into X coordinates on the Canvas.
X(BMI) = (BMI/(maxBMI-minBMI))*Canvas1.Width
That color spectrum would be a series of 8 fat lines of appropriate colors from the low X to the high X of each BMI range.
There is a block to draw a polygon for that triangle, and another block to draw text.