Can you include a screenshot? Airtable can set coloring at the record level, but it cannot color code individual fields. 

Also, are you on a free plan or a paid plan? Paid plans have access to more colors than free plans. 

I'm on a 'Pro' plan and when I get to the sixth color, I can no longer define the condition...

I just tested this and managed to go up to 8:

In your screenshot it looks like you've not scrolled down, so perhaps you could try scrolling within that popup?

Thanks for spotting that - I scrolled down and got two more colors!
However, a few more colors would not go amiss, as I have a dozen variables in this field.