you mean something like ? :
To elaborate on to why there might not be something like .alpha but with tinit
changing the alpha of the image is on each colour, so .rgba, so red green blue alpha. Changing the a of each value would just be setting one variable for each colour in a image.
Where as adding a tint you would have to interpolate the grey and then add the colour on top of it. Er something like that.
Heres the thread i made on this a similar topic a while ago: