import scipy

residues = 123

for i in range(residues):
    bfactor = scipy.linspace(-8,2,residues)[i]
    if bfactor<0: # -8 --> 0
        c = 1+bfactor/8
        color = [1,c,c]
    else:  # 0 --> 2
        c = 1-bfactor/2
        color = [c,c,1]

    cmd.set_color('c%i' % (i),color)
    cmd.color('c%i' % (i),'resi %i' % (a[i,0]))