Table calculation showing increase from min to max

Hi!

I'm not so familiar with table calculations and want to ask some help from Tableau community.

I've made a workbook which shows increasing\decreasing of message amount by "action date" per every "enrty date" (i.e. by each column):

I used this type of table calculation:

I gain the result I expected to see, but I have two problems with it:

1. Why Tableau colors NULL values? Is there any way to make them white?

2. Can I make every maximum value for each "entry date" (column) to start from green? Because now maximum value fades to red with every entry date: the fewer values in group, the more red it becomes.

Is the way I've made table calculation is correct and this is just inevitable Tableau behaviour, or is there more appropriate way to achieve what I want?

I hope that I speak out my mind clear and you can give me an advise, because I was failed to find something similar around here.

Thanks everyone for reading.