@Gratje Home Assistant has recently employed a developer to look at the user interface design and ease of access for all. If you open a discussion topic here: https://github.com/home-assistant/frontend/discussions it will be seen by the developer. They are very keen to hear information about these sorts of issues.
You can also chat with them directly on Discord here: https://discord.gg/KXshVBv5 It is a development channel so keep the discussion to the point.
Note: until an easier way is developed you can change these colours by adding the following lines to a theme and changing the colours that they are currently set to:
energy-grid-consumption-color: "#488fc2"
energy-grid-return-color: "#8353d1"
energy-solar-color: "#ff9800"
energy-non-fossil-color: "#0f9d58"
energy-battery-out-color: "#4db6ac"
energy-battery-in-color: "#f06292"
energy-gas-color: "#8E021b"
You can then use the reload themes service in Developer tools / services to apply your changes.