My Dream Shader

Let me tell you all about my dream shader, and the futility of my efforts to obtain or create it. My sights are set pretty low, it's nothing revolutionary - but it would seem noone else has the same need for this set of functionality as I do.

My wish list:

1) Simple, fast and iOS friendly.
2) Allow 2 definable colours - light and shade - no fancy blending, multiplying or light colour influence. What you choose it what you get.
4) A hard edge 'toon' style terminator/falloff
5) light angle should still be linked to either a directional or point light.
6) Optional textures are multiplied over the top (white = no change)
7) Alpha transparency through both a colour picker (for scripting), and the texture's alpha channel (for sprites)

Here's a quick diagram to illustrate -

My question are, firstly - is this possible, or should I just give up now and go home. And secondly - does anyone know of such a shader, or be willing to make any suggestions to help me figure it out?

I'm a complete novice when it comes to shaders, as you can probably tell, but I'm willing to dive head first into writing one from scratch if that's whats needed. I just need a few pointers to help get me started.

Many thanks!

Harry