Applescript : generate a new list based on existing list

If you're wanting myNewList to return a list of lists I think you're trying to do:

set theList to {"Demo", "ok", "blabla", "demo2"}
set ColortheList to {"5cdf64", "FFFF00", "FF0080", "FF1000"}
set myNewList to {}

repeat with a from 1 to length of theList
    copy ({item a of theList, item a of ColortheList}) to the end of the |myNewList|
end repeat

return myNewList

A little fun with random number. You can modify the color selection and then randomly add to a list:

set theList to {"Demo", "ok", "blabla", "demo2", "foobar"}

set colorList to {"5cdf64", "FFFF00"}
set colorLength to length of colorList

set myNewList to {}

repeat with a from 1 to length of theList
    set colorPick to random number from 1 to colorLength
    copy ({item a of theList, item colorPick of colorList}) to the end of the |myNewList|
end repeat

return myNewList

If you're wanting a string approach try:

set theList to {"Demo", "ok", "blabla", "demo2"}
set ColortheList to {"5cdf64", "FFFF00", "FF0080", "FF1000"}
set myNewList to ""

repeat with a from 1 to length of theList
    set result to ({item a of theList, item a of ColortheList})
    set myNewList to myNewList & (result & " ")
end repeat

The above can be modified in the result. Let me know if I'm understanding you or if you're needing something different.