About one in twelve men and one in two hundred women have some form of colour vision deficiency. In osu! that mostly affects combo colours, approach circles and hit bursts. The good news: a few skin choices fix most problems.
Red/green pairs are the classic trap. Instead of relying on hue, pick colours that also differ clearly in lightness. A safe four-colour set in skin.ini:
[Colours]
Combo1: 0,114,178
Combo2: 230,159,0
Combo3: 240,240,240
Combo4: 204,121,167
This is based on the widely used Okabe–Ito palette, which stays distinguishable for the common types of colour blindness.
Note that many maps define their own colours. Turn on Ignore beatmap skin / colours in options if you want your safe palette everywhere.
approachcircle.png is tinted with the combo colour. A thick, bright outline helps; a thin dark ring on a dark circle can vanish completely.
Don't rely on colour to tell a 100 from a 50. Use bursts with clear numbers or shapes — a miss "X" in white with a dark edge is readable for everyone.
Set SliderBorder to white or a light grey and keep the track darker (SliderTrackOverride). The contrast between border and track then carries the shape, not the hue.
Free simulators (for example the colour-blindness filters in GIMP or many browser extensions) let you view a gameplay screenshot as protanopia, deuteranopia and tritanopia. If two combo colours look the same, change one.
You can start from a clean, minimal skin from osuskins.org and edit its skin.ini; the install guide shows where the skin folder is, and the skin making notes list the colour keys.
Notes by ZentaoLabs. Skin library: osuskins.org.