Is it possible to manually update the value of a Behaviour? (Functional Reactive Programming, Threepenny)

I really hope I haven't gone down a dead-end here. I have a Behaviour that gives the currently selected Color, and the current mouse coordinates, then carries out a task when the mouse is clicked. That task involves looking at a list and then updating the values in that list, for it to be retrieved later. The fact that I can "store" the selected color gives me hope that storing a list can be done in a similar manner. I'm just at a dead end and not sure how to solve this. Would really appreciate some help.

-- There is a Blue button and a Red button on our UI. Whichever
-- button was clicked last is our current color selection.
colorRedSelected = const ColorRed <$ redButton
colorBlueSelected = const ColorBlue <$ blueButton

-- we combine both the above Events to create a new one that tells us the current selected color
colorSelected = unionWith const colorRedSelected colorBlueSelected

-- accumulate values for our Behaviour, starting with ColorRed selected by default
colorMode       <- accumB ColorRed modeEvent

-- create a Behaviour
mouseCoordinate   <- stepper (0,0) $ UI.mousemove canvas

-- want to start with the list [1,2,3,4], but this value should change later.
-- I have 'never' here, as I don't know what else to put here yet.

listState      <- accumB ([1,2,3,4]) never

-- Combine the Behaviours, we now have a tuple (chosenColorMode, mouseCoordinateTuple, savedList)

let choices = (,,) <$> colorMode <*> mouseCoordinate <*> listState

-- Apply the event (of the user clicking the canvas) to the Behaviour,
-- creating a new Event that returns the above tuple when it fires    

makeChoice = choices <@ canvas

onEvent makeChoice $ \(colorMode, (x,y), savedList) -> do    
    -- in this block we use the savedList, and generate a newList.
    -- I want to update the choicePosition behaviour so that the newList
    -- replaces the old savedList.
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