Add code to your extra.less template:
/* Coloured Node Categories */
.block.block--category.block--categoryX .block-header {
background: #009ACF;
color: #ffffff;
}
/**********/
Change X to your node ID. How do I find node ID?
It's simple to find it. Just hover on your Node Category Title like in example below and check for the ID:
So the code for example above will be:
.block.block--category.block--category21 .block-header {
background: #7CA430;
color: #ffffff;
}
For each node ID you need to copy/paste the code above, then just edit to suit your needs.