How to make decreasing quadratic curve?

Hi, I am looking a decreaing quadratic of red colour in the image below. But when I script, I get the curve (in blue). Can someone help me to fix this?

clear all; close all; clc;

fs = 10200; % sampling frequency
dt = 1/fs;
T = 26; % total time
tVec = dt:dt:T; % time vector 
rpm2 = 4500; f2 = rpm2/60;
rpm3 = 0; f3 = rpm3/60;
beta = (f3-f2)/T.^2; % slope rate;
finst = f2 + beta*tVec.^2.';
Speed = finst * 60;

figure()
plot(tVec, Speed)
grid on; box on;

Answer 1

[DGM]

Are you after something like this?

fs = 10200; % sampling frequency
dt = 1/fs;
T = 26; % total time

rpm2 = 4500; f2 = rpm2/60;
rpm3 = 0; f3 = rpm3/60;

tVec = dt:dt:T; % time vector 

beta = (f3-f2)/T.^2; % slope rate;
finst = f3 - beta*(T-tVec).^2.';
Speed = finst * 60;

plot(tVec, Speed)
grid on; box on;

Answer 2

[Image Analyst]

Well your formula just doesn't do that. But here's something closer. At least I have the colors of the lines changing.

all_rpm2 = linspace(4500, -1000, 10);
numCurves = numel(all_rpm2)

numCurves = 10

% Create red colormap
cmap = zeros(numCurves, 3);
cmap(:, 1) = linspace(0.2, 1, numCurves)';
fs = 10200; % sampling frequency
dt = 1/fs;
T = 26; % total time
tVec = dt:dt:T; % time vector
for k = 1 : length(all_rpm2)
    rpm2 = all_rpm2(k)
    f2 = rpm2/60;
    rpm3 = 0;
    f3 = rpm3/60;
    beta = (f3-f2)/T.^2; % slope rate;
    finst = f2 + beta * tVec.^2.';
    Speed = finst * 60;
    plot(tVec, Speed, 'Color', cmap(k, :), 'LineWidth', 3)
    grid on;
    box on;
    hold on;
end

rpm2 = 4500

rpm2 = 3.8889e+03

rpm2 = 3.2778e+03

rpm2 = 2.6667e+03

rpm2 = 2.0556e+03

rpm2 = 1.4444e+03

rpm2 = 833.3333

rpm2 = 222.2222

rpm2 = -388.8889

rpm2 = -1000