Excel VBA highlighting macro
I'm going to talk about variable naming. I can't read your code and understand what's happening which indicates that your code isn't self-explanatory. One step in accomplishing that is to give variables meaningful names:

lastRow is good! Otherwise..

prods - what is this? a dictionary of product keys? why not productList or something similar?

prod how is this different than prods?  Should it be productListKey?

tRange, rv, c, i and tmp - I have no idea what they should be doing - except for i because it's pretty standard.

cStart, currVal and i - you never use them. But if you did  - why not just use the entire word for the description? currentValue and cellStart

Speaking of currVal, cStart, i and tmp - When you don't define your variable, VBA will declare it as a Variant, which are objects:


  Performance. A variable you declare with the Object type is flexible
  enough to contain a reference to any object. However, when you invoke
  a method or property on such a variable, you always incur late binding
  (at run time). To force early binding (at compile time) and better
  performance, declare the variable with a specific class name, or cast
  it to the specific data type.


By not declaring variables, you could possibly be paying a penalty.

What is rv anyway?

Also you are using (rng as Range) in your function - but you're passing it ByRef by default. I don't see any need to do that, so ByVal testRange as Range would be better.



Logic

As far as I can tell you make a dictionary of all values with an interior color. Then you make a dictionary of all values `in the same range as the first dictionary. Then you compare the dictionaries. Since you're already looping through the range for the second dictionary, I'm not sure what kind of speed gains you're getting considering you could just loop through once.

As for finding unfilled cells - I think they are the same. The color is more accurate than colorindex. I'd still use the color property though as all the rest of my code would use color before colorindex.