noStroke()
colorMode(RGB, 100)
for (i in 0:100) {
    for (j in 0:100) {
        stroke(i, j, 0)
        point(i, j)
    }
}

# If the color is defined here, it won't be affected by the
# colorMode() in setup().  Instead, just declare the variable here
# and assign the value after the colorMode() in setup() color bg =
# color(180, 50, 50); # No

bg <- NULL

setup <- function() {
    size(100, 100)
    colorMode(HSB, 360, 100, 100)
    bg = color(180, 50, 50)
}

draw <- function() {
    background(bg)
}

noStroke()
colorMode(HSB, 100)
for (i in 0:100) {
    for (j in 0:100) {
        stroke(i, j, 100)
        point(i, j)
    }
}