You can use this css snippet. There are quite a collection from it came from (links below). If you’re using dark theme change the .theme-light to .theme-dark
.theme-light .graph-view.color-arrow { color: #5cc863; }
.theme-light .graph-view.color-fill-tag { color: #440154;}
.theme-light .graph-view.color-fill-attachment { color: #277f8e; }
.theme-light .graph-view.color-fill-unresolved { color: #fde725; }
.theme-light .graph-view.color-fill { color: var(--background-secondary); }
.theme-light .graph-view.color-circle { color: var(--text-normal); }
.theme-light .graph-view.color-line { color: var(--background-modifier-border); }
.theme-light .graph-view.color-text { color: var(--text-normal); }
.theme-light .graph-view.color-fill-highlight { color: var(--interactive-accent); }
.theme-light .graph-view.color-line-highlight { color: rgb(var(--interactive-accent-rgb)); }
obsidian-css-snippets/Graph nodes.md at develop · Dmitriy-Shulha/obsidian-css-snippets (github.com)