Use consistent colours for the same build

For example, for this mutator perf regression [page](https://github.com/mmtk/ci-perf-result/blob/9c3e127c971a1e34269b9f44245f7d9f800b02c3/master/mutator_history.html), the same build `jdk-mmtk-nogc` is showing up as many different colours. The legend uses the colours from the first graph.