Generating functions and plotting them with different colors

edit

a=1.
b=0.2 
f(c,d)= a / (1 + b*e^(-(x-c)/d))  
sum([plot(f(c,d),[x,0,5],color=hue((c+10*d)/3)) for c in srange(0.2,1.3,.2) for d in srange(0.2,1.3,.2)]).show()

#It seems that 36 curves is too many but I hope it can help to experiment

Or using the suggestion by kcrisman

a=1.
b=0.2 
f(c,d)=a/(1+b*e^(-(x-c)/d)) 
curves=[f(c,d) for c in srange(0.2,1.3,.2) for d in srange(0.2,1.3,.2)]
l=len(curves)
colors=rainbow(l)                                            
sum(plot(curves[k],[x,0,5],color=colors[k]) for k in range(l)).show()