Tinting image problem

I wrote the following script:

var_colors = { "FFFFFF", "FF0000", "00FF00", "0000FF" }
colorIndex = 0
var_currentColor = var_colors[colorIndex + 1]

function switchColor()
colorIndex = (colorIndex + 1) % #var_colors
var_currentColor = var_colors[colorIndex + 1]
end

And I specified the variable var_color as a breached image's tint parameter and a script for invoking switchColor() as its tap action parameter.
Then the image color would change to "0000FF" (blue) from "FFFFFF" (white) , and then back to "FFFFFF", by tapped.
But in fact the color didn't change to white again. It kept blue.
When I did the same doing to a shape, its color correctly changed.