Unsimultan​eously-cha​nging color semi-transparent surfaces

Hello, can anyone help me? I'd like to graph two semi-transparent spheres changing periodically, discreetely and simultaneously colors from red to black to blue according to the sign of cos(t), where t is flowing time in a for loop but with my following code the two spheres change coloros not simultaneously if facealpha is not equal to 1 (or very closely to 1 values like 0.999), for example if facealpha = 0.2 (I need it working with facealpha = 0.2).

[x,y,z]=sphere();
facealpha=0.2;
for i=0:1000
    psi=cos(i);
    hold on
    if psi>0
        surf(x,y,z,'Facealpha',facealpha,'EdgeColor','none','Facecolor','red')
        surf(x,y,z-5,'Facealpha',facealpha,'EdgeColor','none','Facecolor','red')
    elseif psi<0
        surf(x,y,z,'Facealpha',facealpha,'EdgeColor','none','Facecolor','blue')
        surf(x,y,z-5,'Facealpha',facealpha,'EdgeColor','none','Facecolor','blue')
    elseif psi==0
        surf(x,y,z,'Facealpha',facealpha,'EdgeColor','none','Facecolor','black')
        surf(x,y,z-5,'Facealpha',facealpha,'EdgeColor','none','Facecolor','black')
    end
    view([45 45])
    hold off
    pause(0.1)
end

Answer 1

[DGM]

You're trying to create 2002 surface objects. It's not that they're timed wrong; it's just that you've nuked yourself with graphics lag. Create two objects and update them. Don't create new ones each time.

[x,y,z] = sphere();
facealpha = 0.2;

for k = 0:1000 
    psi = cos(k);
    if psi > 0
        facecolor = 'red';
    elseif psi < 0
        facecolor = 'blue';
    elseif psi == 0 % this will never be true
        facecolor = 'black';
    end
    
    if k == 0
        % do the figure setup once on the first pass ...
        h(1) = surf(x,y,z,'Facealpha',facealpha,'EdgeColor','none','Facecolor',facecolor);
        hold on
        h(2) = surf(x,y,z-5,'Facealpha',facealpha,'EdgeColor','none','Facecolor',facecolor);
        view([45 45])
        axis equal
    else
        % ... and then just update only the things that change
        h(1).FaceColor = facecolor;
        h(2).FaceColor = facecolor;
    end
    
    pause(0.1)
end

Also, you're taking the cosine of an integer, which seems odd. Even if it weren't an integer, the numeric result will never be equal to zero.

psi = cos(pi/2) % not zero

psi = 6.1232e-17

You'd have to take into consideration some tolerance in your conditions if you wanted to do this.

[x,y,z] = sphere();
facealpha = 0.2;
tol = eps(1); % or something

for k = 0:1000 
    psi = cos(k);
    if psi > tol
        facecolor = 'red';
    elseif psi < -tol
        facecolor = 'blue';
    else
        facecolor = 'black';
    end
    
    % ... and so on
end