Create new variable based on factor levels and random selection

I'm a little stuck trying to use the sample function for my task, which is to sample n random rows from each level of a factor and create a new variable based on this and the value of another variable.

A simplified example:

Subject = c("100","100","100","100", "100", "200", "200", "200", "200", "200")
Condition = c("Blue","Blue","Blue","Blue", "Blue", "Blue", "Blue", "Blue", "Blue", "Blue")
Response = rnorm(10)
df = data.frame(Subject,Condition, Response) 

For instance, a goal would be to sample 3 random rows for each level of Subject, create a new variable, let's say Condition.Rand which has the randomly selected rows labeled "Red" and the remaining ones labeled with whatever value was in Condition -- in this case, "Blue". Thus for each Subject, 60% of Condition.Rand would be labeled "Red" and 40% labeled "Blue".

Thank you!