The fastest algorithm I could find that finds the chromatic number of an undirected simple graph exactly in only polynomial-space is "Faster Graph Coloring in Polynomial Space" by Gaspers and Lee (DOI: 10.1007/978-3-319-62389-4_31). It's running time is $O(2.2356^n)$.
However that's from 2016/2017, so I'm wondering whether there are any new relevant developments.
The mentioned algorithm is based on a procedure for counting the number of independent sets of a graph, meaning that new developments in that area are possibly relevant, too.