One size up, e.g. if you are already using a 55mm filter, you will need to use a 58mm filter if you need to overlay a filter 

As K&F Concept ND2-ND400 filters adjusts the amount of light that can pass through with polarizing glass, a colour cast can't be avoided for all the variable ND filters on the market. However, K&F Concept variable ND filters are made by the best coating technology, which reduces the effect of the polarizing film on image color. *Based on our professional test, the colour cast of K&F Concept ND2-ND400 filters is around 290k while the industry standard is 500k. 

Looking at the back 4 buttons. The button on the far right - press and hold power comes on. All digits come on. Two right hand digits show up to 99. These are the brightness, the two left digit are the temperature of the light. If you press the two buttons on the left, then the intensity goes up and down. Tap the second button from the right, press and the right two number flash. Use the two buttons on the left to change the value up and down. Colour temperature changes from 28 -60. Low value is warmer (more cream) , higher value is colder (more blue). That's all there is to it. 

I don't know if it's an upgraded version but any colour cast is minimal. 

Yes, they are stackable.