Multimode features somewhat complex controls. Here's a guide to crafting a virtual world:
To change the Hue, or color, press U and J.
To change the Saturation, or grayness, press I and K.
To change the Luminosity, or lightness, press O and L.
To cycle through the block textures, press Q and E.
To switch between wall blocks and solid blocks, press space. You should see the black outline turn white and black.
To choose to place either building blocks or light blocks, press P and ;.