Blue

Let R be the answer to the red problem, B is the answer to the blue problem and W is the answer to the white problem.

Then using the information in each problem we can form the following simultaneous equations:

R = B - 21

B = 2W

W = 4R

Then solving simultaneously, the last two equations tell us that B = 8R. Substituting this into the first equation, we get R = 8R - 21 so R = 3, B = 24 and W = 12.

The only combination of answers that works consistently for all three of these questions is:

• Red: $3$
• White: $12$
• Blue: $\boxed{24}$