Here are some visual design rules you can learn once and use many times in your career. They’re safe.
Pure black looks unnatural on a screen, and pure white is too bright. Use a vey dark and very light grey, respectively. Any other references to “black” and “white” in these rules assume you’re following this rule.
A neutral is generally a black, white, or grey. If you use colour in your interface, add a little bit of that colour to your neutrals. If you use the HSB colour system (and you should) less than 5 Saturation should do it.
Important elements means buttons, content, or anything else that the user needs to notice. Elements that the user does not need to notice (e.g. structural elements, drop-shadows) can use as little contrast as possible.
This is vague, but very important. You should be deliberate about absolutely everything in your design. This means whitespace, alignment, size, spacing, colour, shadows. Everything. If I point at a random part of your design and you don't have an explanation for why it looks that way, you’re not finished.
Mathematical alignment is when you tell your design software to centre something in its container and think you’re done. But some shapes don’t suit being aligned in this way. Very often you will need to align things by eye so that it looks good.
This applies to all text. The bigger the text, the less space you need between each letter and each line. The reverse is also true.
Example: If you have a card with a 1px border and a dark background, and it sits on top of an even darker background, the 1px border should be lighter than both of them. It should not be set to a brightness somewhere between the card and page background colours. The same applies to light background colours: the 1px border should be darker than both background colours.
If something is not aligned with anything else in the interface, it looks terrible. Ideally each element will be aligned with other elements based on some kind of logic.
It’s OK if some colours in your palette share the same brightness value, but if a colour has a different brightness value it should be noticeably different.
Having some neutrals with a little bit of warm colour and some with cool colours looks bad.
The spacing you use between elements, and the size of elements, should be determined by some kind of scale. This will help the design to look consistent. An easy way to do this is a column grid and a baseline grid.
If you have a series of elements in a row or column, and some are more visually heavy than others (two buttons and three links, for example), you should arrange them like a triangle. The visually heaviest element should go first, and the least heavy element last, in order. One caveat is that the visually heaviest element should be on the outside edge. If your elements are against the right edge of the design, for example, the heaviest element should be against the right edge.
If you’re going to break your design up into vertical columns, use 12 columns. A 12 column grid can be broken up into 1 column, 2 columns, 3 columns, and 4 columns, so it gives you a lot of flexibility.
When you’re measuring out space between elements in a design—for example if you want 100px of vertical space between blocks of content on a landing page—the spacing should be from one point of high contrast to the next.
A white background with black paragraphs of text means that the points of contrast will be the end of one paragraph and the start of the next. But if you put a black background behind one white paragraph, the spacing should run from the end of one paragraph to the start of the black background, then again from the start of the black background to the start of the paragraph.
As elements on the screen get closer to the user, they should get lighter. This applies to both light and dark mode UIs.
e.g. If you create a shadow which extends 4 pixels on the Y axis, use a blur value of 8 pixels. As the element gets “closer” to the viewer, it’s a good idea to also lower the opacity of the shadow.