Apparently some people are of the opinion that “rustic” concrete planters means making a simple yet ugly form, mixing up some instant concrete mix, dumping it in without even joogin’ it down, and then removing the form a couple/three days later and revealing it in all its ugly glory. STOP THAT RIGHT NOW!
While I admit that I am a practitioner of “anything that’s worth doing well can be done badly much more quickly, easily, and cheaply”, concrete ain’t one of those things! It is so easy to do well, why not do it right from the beginning? There are recipes for various concrete mixes readily available online. You can purchase wet pigments to add to the (wet) mixture, or dry pigments to mix into the dry mixture before adding water. Personally, I like to oil whatever form I’m using, then puff the dry pigment (various colors) onto the oiled form, then press the concrete mix (I like mine pretty dry) into the pigment on the wall of the form for more interesting color. For the acrylic admixture (bonding agent) called for in some concrete recipes, I use….paint. Yep, the very acrylic paint that you will find in the oopsie, somebody done messed THAT color up aisle of the paint store or hardware store. Just remember that your cement color is usually gray unless you got all fancy and purchased a nice white portland cement. Since your cement is gray, the color (say you got a garish bright blue) will be a toned down grayish blue.