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Here is one way to use a colormap with your plots.

In this example, we set the number of curves, and run through the colormap in that many steps.

sage: cm = colormaps.Blues
sage: n = 20
sage: color = lambda j: cm(floor(j*255/n))[:3]
sage: Terms = [x^(2+j)*(2+(-1)^(j+1)) for j in range(n)]
sage: Sums = [sum(Terms[0:n]) for n in range(n)]
sage: Figures = [plot(Sums[j], (x, -1.2, 1.2), ymin=-3, ymax = 3, color=color(j)) for j in range(n)]
sage: f = sum(Figures)
sage: f.show()

Here is one way to use a colormap with your plots.

In this example, we set the number of curves, and run through the colormap in that many steps.

sage: cm = colormaps.Blues
sage: n nmax = 20
sage: color = lambda j: cm(floor(j*255/n))[:3]
cm(floor(j*255/nmax))[:3]
sage: Terms = [x^(2+j)*(2+(-1)^(j+1)) for j in range(n)]
range(nmax)]
sage: Sums = [sum(Terms[0:n]) for n in range(n)]
range(nmax)]
sage: Figures = [plot(Sums[j], (x, -1.2, 1.2), ymin=-3, ymax = 3, ymax=3, color=color(j)) for j in range(n)]
range(nmax)]
sage: f = sum(Figures)
sage: f.show()

Here is one way to use a colormap with your plots.

In this example, we set the number of curves, and run through the colormap in that many steps.

sage: cm = colormaps.Blues
sage: nmax = 20
sage: color = lambda j: cm(floor(j*255/nmax))[:3]
sage: Terms = [x^(2+j)*(2+(-1)^(j+1)) for j in range(nmax)]
sage: Sums = [sum(Terms[0:n]) for n in range(nmax)]
sage: Figures = [plot(Sums[j], (x, -1.2, 1.2), ymin=-3, ymax=3, color=color(j)) for j in range(nmax)]
sage: f = sum(Figures)
sage: f.show()

Edit. (To answer follow-up question in the comment.)

Colormaps have 256 levels numbered 0 to 255, so color above maps [0 .. nmax - 1] into [0 .. 255].

To leave out the first half of the colormap, use instead:

sage: color = lambda j: cm(floor(128 + j*127/nmax))[:3]

or if you want to go nonlinearly using some function h from [0 .. nmax-1] to the interval [0, 1], just use

sage: color = lambda j: cm(floor(255*h(j))[:3]

(That looks better indeed.)

The reason for [:3] is that a colormap returns four values, corresponding to R, G, B and opacity (also known as alpha), but typically opacity is 1.0 throughout (since most colormaps do not play with transparency).

To further refine the above and make the transition through colors more continuous, instead of using cm(floor(...)) one could use the fractional part of 255*h(j) to move gradually between the colors at floor(255*h(j)) and ceil(255*h(j)) in the colormap... (Not clear one could perceive the effect of that though!)

To do that, given a function h as above, you could do

def color(j):
    h = RR(255*h(j))
    r0, g0, b0 = cm(h.floor())[:3] # rgb at floor
    r1, g1, b1 = cm(h.ceil())[:3] # rgb at ceiling
    f = h.frac() # fractional part
    r = (1-f) * r0 + f * r1
    g = (1-f) * g0 + f * g1
    b = (1-f) * b0 + f * b1
    return r, g, b

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