The portfolios I trust most have one accent color. Usually a desaturated blue, sometimes a deep green, occasionally a low-key crimson. The rest is ink on paper.
The reason is almost embarrassingly simple: one accent forces you to be careful about where you put it. Two accents lets you off the hook. Three accents and you've left the building.
What one accent buys you
It buys a hierarchy that doesn't need explaining. The accent is the eye's gravity. Anywhere it lands is "this matters." Everything else is "this is context." A reader can navigate the page at one glance.
What one accent costs you
It costs the ability to color-code. If the accent has to mean "important," then it cannot also mean "warning," "active," or "selected." Those have to be done with weight, position, and motion — which is harder, but better, because those tools survive being printed in black and white.
When to break the rule
When you're designing a status surface — health dashboards, monitoring, notifications — color-coding earns its keep. But on a portfolio? On a memo? One accent is enough. One accent is the discipline.