Re: What is responsible at the cellular level for red and white colors of yeast

When yeast is deficient in the Ade2 gene product then it accumulates 
an intermediate which is a red pigmented substance*. If you have an 
excess of adenine in the medium, then the strain grows fine and the 
ADE genes are not expressed and don't accumulate  the 
intermediate. When adenine is limiting then the ADE genes are 
expressed and you accumulate this red pigmented compound. 

The genotype of the strain you list is ade2 and ade3 minus, but the 
ade3 defect is complemented by ADE3 on the plasmid, so in effect it is 
just ade2 defective. 

So what you are observing is that the adenine is being used in the 
patch of dense growth and those cells are now producing pigment. If 
you increase the amount of adenine in the plate they should all stay 
white, if you decrease it then the colonies should all begin to turn red. 

*Moderator's footnote:
The red pigment is 5-aminoimidazole ribonucleotide (AIR).  The product
of the ade2 yeast gene is an enzyme that modifies AIR and is called
AIR carboxylase.