GPS: This is a fun script that makes gradients:
#! /usr/local/bin/wish8.3 set im [image create photo -width 600 -height 768] for {set i 1} {$i < 256} {incr i} { set col [format "%2.2XFFFF" $i] $im put "#$col" -to 0 $i 600 [expr {$i + 1}] } for {set i 1} {$i < 256} {incr i} { set col [format "FF%2.2XFF" $i] set yi [expr {$i + 256}] $im put "#$col" -to 0 $yi 600 [expr {$yi + 1}] } for {set i 1} {$i < 256} {incr i} { set col [format "FFFF%2.2X" $i] set yi [expr {$i + 512}] $im put "#$col" -to 0 $yi 600 [expr {$yi + 1}] } pack [canvas .c -bd 0 -height 768 -width 600] .c create image 300 384 -image $im -tag im .c bind im <Button-1> {puts [$im get %x %y]}
uniquename 2014jan27
For those who do not have the facilities or time to implement the code above, here is an image that shows the single Tk 'photo' image put on a Tk 'canvas'.
For another method of making color-gradients in rectangles, see the wiki page Making color gradients. The technique there uses stacks of many thin Tk 'frame' widgets and gradiates the background color of the frame widgets.