Mixing clear acrylics?

I tried mixing Tamiya clear yellow & blue to make a blue-green, 'thinned' with future, that was ok,

Then I tried Vallejo dark blue with future, sprayed ok, but tiny spots of blue instead of even colour, not good

Thought about using waterproof felt-tips? 

I have used magic markers for tinting lenses with some success.  Some are very transparent and allow you to build up as much color as you want with a number of applications. I would dry markers on scrap to make sure you like the color and effect.

If you want tinted clear colors you can make your own with a simple formula that is both cheap and easily available. Future mixed with food coloring. 1 to 2 drops of food coloring to 1 oz of Future.

That's how I do it when not using vallejo transparent colors!