Whisk together flour and salt. In a large bowl, using a mixer, beat butter and sugar on medium-high until pale and fluffy, 4 minutes. Beat in eggs, one at a time, then vanilla. Reduce speed to low, add flour mixture, and beat until combined. Tint dough by mixing in food coloring: For trees, tint two-thirds the dough green; tint some of remaining dough red and some yellow; leave some plain. Divide green dough into 2 disks. Individually wrap all dough in plastic and refrigerate 1 hour (or up to 3 days).