The 1950 edition of The Officer's Guide includes the notation that the pre-war blue uniforms are authorized for wear on an optional basis, and shows an image of an officer in the full-dress uniform (because the officer is wearing the sword belt, but not the saber)
They mean business
Stick to tradition, avoid bright colors, and, when in doubt, dress like you would for a job interview
However, if your beige dress is somewhat shiny or metallic, you should definitely check out my post on what shoes to wear with a champagne dress, instead
Dessy Group Jennifer Desert Rose Matte Satin Bridesmaid Dress | Birdy Grey Birdy Grey AZAZIE XYLA - All Dresses Final Sale Bridesmaid Dresses Shop Azazie Xyla Auburn Bridesmaid Dress in Metallic Satin starting at $69
Think of these guidelines as just that, a guide, it doesnt mean you have to stick to it, its just there to give you an idea