Linear Gradient

The lerpColor() function is useful for interpolating between two colors.

from p5 import *

Y_AXIS = 1
X_AXIS = 2
b1 = b2 = c1 = c2 = None

def setup():
        size(640, 360)

        global b1, b2, c1, c2
        b1 = Color(255)
        b2 = Color(0)
        c1 = Color(204, 102, 0)
        c2 = Color(0, 102, 153)

        no_loop()


def draw():
        global b1, b2, c1, c2

        # background
        setGradient(0, 0, width/2, height, b1, b2, X_AXIS)
        setGradient(width/2, 0, width/2, height, b2, b1, X_AXIS)
        # Foreground
        setGradient(50, 90, 540, 80, c1, c2, Y_AXIS)
        setGradient(50, 190, 540, 80, c2, c1, X_AXIS)

def setGradient(x, y, w, h, c1, c2, axis):
        no_fill()

        if axis == Y_AXIS:
                for i in range(y, y + int(h) + 1):
                        inter = remap(i, [y, y+h], [0, 1])
                        c = c1.lerp(c2, inter)
                        stroke(c)
                        line((x, i), (x+w, i))
        elif axis == X_AXIS:
                for i in range(int(x), int(x + w) + 1):
                        inter = remap(i, [x, x+w], [0, 1])
                        c = c1.lerp(c2, inter)
                        stroke(c)
                        line((i, y), (i, y+h))

if __name__ == '__main__':
  run()