Hi,
As per the understanding of the question, you would like to add different background color for area above and area below the plotted line.
This can be done using the 'area' function in MATLAB. Here is an example for the same.
% Define constants
a = 0.1; % adjust this value to change the shape of the parabola
% Define range of y values
y = linspace(-10, 10, 100);
% Calculate corresponding x values
x = a * y.^2;
area(x)
You can explore more about the 'area' function by referring to the following link:
I hope this helps.
