Find RMSE of mvregress outputs

Hi,

I am trying to use multivariate regression on three color channel data using mvregress.

data = [red;green;blue]; % this is a 3x20 matrix
Y = data';
X = requiredOutput';  % requiredOutput is a 1x20 vector
[beta,Sigma, resid] = mvregress(X,Y)

After performing this, I need to find RMSE for this regression.

How can I do this?

Thanks in advance.