You can apply various color adjustments to a loaded image, then render it to the canvas.  You can also grab colors from an image for use in a gradient or to paint objects on the canvas.

#!/usr/local/bin/macruby

framework 'cocoa'
require 'rubygems'
require 'hotcocoa/graphics'
include HotCocoa
include Graphics

# set up the canvas and font
canvas = Canvas.new :type => :image, :filename => 'colors.png', :size => [400,400]
canvas.background(Color.white)
canvas.font('Skia')
canvas.fontsize(14)
canvas.fill(Color.black)

# LOAD IMAGE
img = Image.new('v2.jpg')

w,h = [100,100]
x,y = [0,290]
xoffset = 105
yoffset = 130

# ORIGINAL image, resized to fit within w,h:
img.fit(w,h)
canvas.draw(img,x,y)
canvas.text("original",x,y-15)
x += xoffset

# HUE: rotate color wheel by degrees
img.reset.fit(w,h)
img.hue(45)
canvas.draw(img,x,y)
canvas.text("hue",x,y-15)
x += xoffset

# EXPOSURE: increase/decrease exposure by f-stops
img.reset.fit(w,h)
img.exposure(1.0)
canvas.draw(img,x,y)
canvas.text("exposure",x,y-15)
x += xoffset

# SATURATION: adjust saturation by multiplier
img.reset.fit(w,h)
img.saturation(2.0)
canvas.draw(img,x,y)
canvas.text("saturation",x,y-15)
x += xoffset

# (go to next row)
x = 0
y -= yoffset

# CONTRAST: adjust contrast by multiplier
img.reset.fit(w,h)
img.contrast(2.0)
canvas.draw(img,x,y)
canvas.text("contrast",x,y-15)
x += xoffset

# BRIGHTNESS: adjust brightness
img.reset.fit(w,h)
img.brightness(0.5)
canvas.draw(img,x,y)
canvas.text("brightness",x,y-15)
x += xoffset

# MONOCHROME: convert to a monochrome image
img.reset.fit(w,h)
img.monochrome(Color.orange)
canvas.draw(img,x,y)
canvas.text("monochrome",x,y-15)
x += xoffset

# WHITEPOINT: remap the white point
img.reset.fit(w,h)
img.whitepoint(Color.whiteish)
canvas.draw(img,x,y)
canvas.text("white point",x,y-15)
x += xoffset

# (go to next row)
x = 0
y -= yoffset

# CHAINING: apply multiple effects at once
img.reset.fit(w,h)
img.hue(60).saturation(2.0).contrast(2.5)
canvas.draw(img,x,y)
canvas.text("multi effects",x,y-15)
x += xoffset

# COLORS: sample random colors from the image and render as a gradient
img.reset.fit(w,h)              # reset the image and scale to fit within w,h
colors = img.colors(10).sort!   # select 10 random colors and sort by brightness
gradient = Gradient.new(colors) # create a new gradient using the selected colors
rect = Path.new.rect(x,y,img.width,img.height) # create a rectangle the size of the image
canvas.beginclip(rect)          # begin clipping so the gradient will only fill the rectangle
canvas.gradient(gradient,x,y,x+img.width,y+img.height) # draw the gradient between opposite corners of the rectangle
canvas.endclip                  # end clipping so we can draw on the rest of the canvas
canvas.text("get colors",x,y-15)    # add text label
x += xoffset

canvas.save

Radial or linear gradients may be drawn by first specifying their main constituent colors, then applying to the canvas with a start and endpoint. (warning: crashes in MacRuby 0.4)

#!/usr/local/bin/macruby

framework 'cocoa'
require 'rubygems'
require 'hotcocoa/graphics'
include HotCocoa
include Graphics

# FIXME: SEGFAULT!!

# set up the canvas and font
canvas = Canvas.new :type => :image, :filename => 'gradients.png', :size => [400,400]
canvas.background(Color.black)

# create a new gradient
gradient = Gradient.new

# set the component colors of the gradient
gradient.set(Color.black,Color.blue,Color.red.darken,Color.orange)

# draw a linear gradient starting at 50,50 and ending at 200,200
canvas.gradient(gradient, 50,50,200,200)

# do not extend gradient beyond its start/endpoints
gradient.pre(false)
gradient.post(false)

# draw a radial gradient starting at 200,200 with radius 100
canvas.radial(gradient, 200,200,100)

# save the canvas
canvas.save

You can load an image, grab an array of colors from it, and then apply those colors to other objects on the canvas.

#!/usr/local/bin/macruby

framework 'cocoa'
require 'rubygems'
require 'hotcocoa/graphics'
include HotCocoa
include Graphics

# set up the canvas and font
canvas = Canvas.new :type => :image, :filename => 'parsing.png', :size => [400,400]
canvas.background(Color.black)
canvas.shadow

# load an image and select 100 random colors
image = Image.new('italy.jpg')
randomcolors = image.colors(100)

# create a 20x20 square at 0,0
square = Path.new.rect(0,0,20,20,:center)

# randomize the color, scale, and rotation of the square
square.randomize(:fill, randomcolors)
square.randomize(:scale, 1.0..5.0)
square.randomize(:rotation, 0..360)

# draw 100 squares and the original image
canvas.grid(square,10,10)
image.fit(120,120)
canvas.draw(image)
canvas.save

Various sets of colors can be generated from a single starting color by using the various classical color harmony schemes.  For more information, see Color Theory on Wikipedia.

#!/usr/local/bin/macruby

framework 'cocoa'
require 'rubygems'
require 'hotcocoa/graphics'
include HotCocoa
include Graphics

# set up the canvas and font
canvas = Canvas.new :type => :image, :filename => 'harmony.png', :size => [400,400]
canvas.background(Color.white)
canvas.font('Skia')
canvas.fontsize(12)

# create a new color with the specified red, green, blue, and alpha values
blue = Color.new(0.0,0.4,0.6,1.0)

# draw the original color
canvas.translate(135,350)
canvas.text("original color",-115,10)
canvas.fill(blue)
canvas.rect(0,0,255,30)
canvas.fill(Color.black)

# create a rectangle to use as a color swatch
swatch = Path.new.rect(0,0,15,30)
swatch.increment(:x, 15)

# draw harmony schemes derived from this color
for scheme in [
  :complementary,:split_complementary,:left_complement,
  :right_complement,:monochrome,:triad,:tetrad,:compound,
  :flipped_compound
  ] do
  canvas.translate(0,-38)
  canvas.text(scheme,-115,10)
  swatch.increment(:fill, blue.send(scheme).sort)
  canvas.draw(swatch,0,0,17)
end

# save the canvas
canvas.save