db_subst doesn't display

[kin boster <kinboster@gmail.com>]

hi all:

       I wrote a standalone program based debconf which is include multiselect and subst, but it doesn't display anything, what's wrong?
       I want to set default value is select all, how can i write in 'Default' field?

       here is my test program:
#!/bin/sh -e

. /usr/share/debconf/confmodule

db_capb backup
mcolor="red, Yellow, green"
db_subst demo/multiselect COLORS "$mcolor"
db_input medium demo/multiselect


scolor=$RET
echo $scolor

db_go


      here is test.templates:
Template: demo/multiselect
Type: multiselect
Choices: ${COLORS}, gray, grey,  orange, maroon, black, white, navy blue, chartruse, mahogony, blue, bluegreen

Description: What is your favorite COLOR?
 This is a demo select list item. The list is generated on the fly,
 and the available choices are: ${COLORS} again, that's: ${COLORS}.
 

some advise?

best regards

boster