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To complement the answer by @achrzesz, let us point out two additional approaches to fill exactly the region between the curves $y=f(x)$ and $y=g(x)$ where $f(x)\leq g(x)$, with $x$ in a given interval $[a,b]$, that is, the region
$$\{(x,y)\in\mathbb{R}^2 \mid a\leq x\leq b,\ f(x)\leq y\leq g(x)\}.$$ First method:

f(x) = sin(2*x+3)
g(x) = 2*cos(2*x+2)
pf = plot(f(x), (x,-3,3), color="red", fill=max_symbolic(f(x),g(x)), fillcolor="lightgreen")
pg = plot(g(x), (x,-3,3), color="blue")
show(pf+pg)

Second method:

var("x,y")
p = plot([f(x),g(x)], (x,-3,3), color=["red","blue"])
r = region_plot([f(x)<y, y<g(x)], (x,-3,3), (y,-2,2), incol="lightgreen", plot_points=[100,100])
show(r+p)

See this SageMath Cell

To complement the answer by @achrzesz, let us point out two additional approaches to fill exactly the region between the curves $y=f(x)$ and $y=g(x)$ where $f(x)\leq g(x)$, with $x$ in a given interval $[a,b]$, that is, the region
$$\{(x,y)\in\mathbb{R}^2 \mid a\leq x\leq b,\ f(x)\leq y\leq g(x)\}.$$ First method:

f(x) = sin(2*x+3)
g(x) = 2*cos(2*x+2)
pf = plot(f(x), (x,-3,3), color="red", fill=max_symbolic(f(x),g(x)), fillcolor="lightgreen")
pg = plot(g(x), (x,-3,3), color="blue")
show(pf+pg)

Second method:

var("x,y")
p = plot([f(x),g(x)], (x,-3,3), color=["red","blue"])
r = region_plot([f(x)<y, y<g(x)], (x,-3,3), (y,-2,2),  incol="lightgreen", plot_points=[100,100])
show(r+p)

See this SageMath Cell

To complement the answer by @achrzesz, let us point out two additional approaches to fill exactly the region between the curves $y=f(x)$ and $y=g(x)$ where $f(x)\leq g(x)$, with $x$ in a given interval $[a,b]$, that is, the region
$$\{(x,y)\in\mathbb{R}^2 \mid a\leq x\leq b,\ f(x)\leq y\leq g(x)\}.$$ First method:

f(x) = sin(2*x+3)
g(x) = 2*cos(2*x+2)
pf = plot(f(x), (x,-3,3), color="red", fill=max_symbolic(f(x),g(x)),  fillcolor="lightgreen")
pg = plot(g(x), (x,-3,3), color="blue")
show(pf+pg)

Second method:

var("x,y")
p = plot([f(x),g(x)], (x,-3,3), color=["red","blue"])
r = region_plot([f(x)<y, y<g(x)], (x,-3,3), (y,-2,2), 
                 incol="lightgreen", plot_points=[100,100])
show(r+p)

See this SageMath Cell