An option to choose the color opacity should be available. The one in Figma is particularly efficient.
Reference: https://help.figma.com/hc/en-us/articles/360041003774-Apply-paints-with-the-color-picker#:~:text=in%20your%20browser.-,Open%20the%20color%20picker,the%20Fill%20or%20Stroke%20section.