Answer:
PP (genotype) - Purple (Phenotype)
Pp (Genotype - Purple (Phenotype)
pp (Genotype) - White (Phenotype)
Explanation:
The genotype allele combinations, in order to for purple to be dominant there are three possible genotypes
*Capital P represents a purple dominant allele
*Lowercase p represents a white recessive allele
(Alleles are the variants of a certain gene, ex: brown eyes, green eyes, etc. ) 
A Dominant always overpowers the recessive, the only way for a recessive to show would be to have two recessive alleles from both parents
ex: Green eyes recessive to Brown 
B - Brown       bb = green eyes (from dad)   
b - green       bb = green eyes (from mom)  
this means that their child would have green eyes because there is no brown present 
 
So for your pea plants to be purple
Pp = Purple      pp = white 
PP = Purple    (yes there can be white recessive present, it just might not appear as a plant because of the percent of purple) 
The colors or observations we see as the plant are the phenotypes
so the colors I have listed would be the phenotypes 
Don't get alleles and phenotypes confused, alleles are the various forms of a gene, and phenotypes are what we see when the gene is brought through.