This was written when using Defold version 1.6.3.
Sending variables to the shader program from a game object script is easy.
go.set("go#some_sprite", "primary_colour", vmath.vector4(0.5, 1, 0, 1))
Besides the above code, your custom material file also needs a Fragment Constant set with identical value: primary_color, type being: User.
Oh and the sprite needs to actually use that material.
You’re done now. 
Oh what about sprites that are used in gui? Well.. Get ready to make your own custom renderer.. Fortunately it is surprisingly easy!
Links that helped me to figure things out:
Relevant code
gui_colour.fp
uniform lowp vec4 primary_colour;
custom.render_script
function init(self)
-- New pridicate which is what needs to be in the materials file "Tags" section.
self.gui_tint_pred = render.predicate({ "gui_colour" })
...
self.constants = render.constant_buffer()
self.constants.primary_colour = vmath.vector4(1, 0, 0, 1)
end
function update(self)
...
render.draw(self.gui_pred, { frustum = frustum })
-- Add the custom material which is set in the custom render file
render.enable_material("gui_mat")
render.draw(self.gui_tint_pred, {
frustum = frustum,
constants = self.constants
})
render.disable_material()
render.draw(self.text_pred, { frustum = frustum })
...
end
function on_message(self, message_id, message)
...
if message_id == hash("change_primary_colour") then
self.constants.primary_colour = message.p
end
end
cake.gui_script
msg.post("@render:", "change_primary_colour", { p = vmath.vector4(0.5, 1, 0, 1) })