Do I understand that you are trying to make a plate with just lactose as C source? Well, you have to add other ingredients like N source unless your yeast is a N-fixer, it cannot get N out of the air, even then pure lactose and agar won’t likely support your yeast… You can use minimal medium and there are some media without C source. There are chemically defined media used to determine the C source requirement of unknown bacteria. You can use that.

hope that helps.

than use some minimal media and as carbon source use lactose.
But bacteria and yeast have quite distinct needs, so already the choose of media will be discriminating 😉

why should it be blue? Do you have some transformed bacteria? What are you doing?

I don’t understand what you are trying to do. If you want to select for yeasts that can metabolize lactose, then do what JackBean said. Use minimal medium + lactose as the sole C source.

Then you said you also wanted to grow bacteria on it? Meaning you have a mixed population at the start? For bacteria, you have to have some idea on its nutrient requirements not just the ability to break down lactose. Another thing is that once lactose is broken by enzymes released in the environment. You’ll get small colonies around it that will also be growing although they cannot metabolize lactose. These small colonies can grow because the lactase enzymes are extracellular thereby resulting to the presence of glucose which almost all bacteria/yeast can utilize.

Historical studies also suggest that bacteria (or maybe even yeast) can "adapt" to the presence of lactose as the sole C source. – http://www.jstor.org/pss/82643