[http://www.xmission.com/~georgeps/math/gradientFun.png]

[GPS]: The code below draws the image above.  It's reasonably quick, and seems to be bug free.
----
  proc + {n1 n2} {
    expr {$n1 + $n2}
  }
  proc - {n1 n2} {
    expr {$n1 - $n2}
  }
  proc * {n1 n2} {
    expr {$n1 * $n2}
  }
  proc / {n1 n2} {
    expr {$n1 / $n2}
  }
  proc toInt {n} {
    expr int($n)
  }
  
  proc drawGradient {win type col1Str col2Str} {
    $win delete gradient
    
    set width [winfo width $win]
    set height [winfo height $win]
    
    foreach {r1 g1 b1} [winfo rgb $win $col1Str] break
    foreach {r2 g2 b2} [winfo rgb $win $col2Str] break
    set rRange [- $r2.0 $r1]
    set gRange [- $g2.0 $g1]
    set bRange [- $b2.0 $b1]
  
    if {$type == "x"} {
      set rRatio [/ $rRange $width]
      set gRatio [/ $gRange $width]
      set bRatio [/ $bRange $width]
    
      for {set x 0} {$x < $width} {incr x} {
        set nR [toInt [+ $r1 [* $rRatio $x]]]
        set nG [toInt [+ $g1 [* $gRatio $x]]]
        set nB [toInt [+ $b1 [* $bRatio $x]]]
  
        set col [format {%4.4x} $nR]
        append col [format {%4.4x} $nG]
        append col [format {%4.4x} $nB]
        $win create line $x 0 $x $height -tags gradient -fill #${col}
      }
    } else {
      set rRatio [/ $rRange $height]
      set gRatio [/ $gRange $height]
      set bRatio [/ $bRange $height]
  
      for {set y 0} {$y < $height} {incr y} {
        set nR [toInt [+ $r1 [* $rRatio $y]]]
        set nG [toInt [+ $g1 [* $gRatio $y]]]
        set nB [toInt [+ $b1 [* $bRatio $y]]]
  
        set col [format {%4.4x} $nR]
        append col [format {%4.4x} $nG]
        append col [format {%4.4x} $nB]
        $win create line 0 $y $width $y -tags gradient -fill #${col}
      }
    }
    return $win
  }
  
  canvas .grad1
  bind .grad1 <Configure> [list drawGradient .grad1 x red royalblue]
  
  canvas .grad2
  bind .grad2 <Configure> [list drawGradient .grad2 y yellow red]
  
  pack .grad1 .grad2 -fill both -expand 1
----
[Category Graphics]