We have defined the residual error as
\boldsymbol{\epsilon}=\boldsymbol{y}-\tilde{\boldsymbol{y}}=\left[\boldsymbol{I}-\boldsymbol{X}\left(\boldsymbol{X}^T\boldsymbol{X}\right)^{-1}\boldsymbol{X}^T\right]\boldsymbol{y}.The residual errors are then the projections of \boldsymbol{y} onto the orthogonal component of the space defined by the column vectors of \boldsymbol{X} .