For the dahlias, there is probably an insect or small caterpillar eating the holes in the leaves and flowers.  Look closely and if you find anything, take a picture and send it in, then we can tell you if you need to treat it or if it's just a temporary problem. 

As for the lantana, from the picture it looks like your soil is very wet.  Use a trowel and dig down a few inches, grab a handful of soil, and squeeze it.  If it drips water or sticks together in a clump, it is too wet.  The soil should feel like a squeezed out sponge and just start to fall apart before watering again. 

As for the color on the lantana, when a tag says multicolored, that just means it's a toss up for what you will get, sometimes, pink, sometimes yellow, sometimes white.  When you buy multiple plants you may end up will all the same color. 

Thank you for your question.