Gradient colors

How would I write a function that would take two colors: (255,0,0), and (0,0,255), a value from 0.0 to 1.0, and have it output the color in between the first two based on that number.  Like, if I ran this function like this:

gradient((255,0,0),(0,0,255),0.5)

it would return


(128,0,128)


r = r1*p+r2*(p-1);
g = g1*p+g2*(p-1);
b = b1*p+b2*(p-1);

...isn't it obvious?