I have found that reducing the opacity against a white background is a good way to find usable lighter and less saturated tints of a base color.
As an example take this picture of an orange color:
The lower row shows the variants. The percentages are the opacities.
Having a white background and reducing the opacity is good when I search good tints, but I like to use equivalent 100% opaque RGB colors in the final product.
How do I calculate or otherwise find the equivalent RGB numbers, when the RGB numbers of the base color and the opacity are already selected?
I have tried color picker, but in Illustrator it at least gives only the base color.
Use formula Y=255 – P*(255-X) where X is a RGB number, P=opacity (0…1), Y=new RGB number which should give the same appearance with 100% opacity as X gives with 100p% opacity against white background.
The formula is the general opacity formula, only simplified for this special case – the partially transparent top layer is against pure white.
Note: the white background should be a white object, not the artboard white. White background object is color managed.
If you are in Illustrator and want to copy the color with the color picker, make a copy of the partially transparent object and rasterize it. Select white background in the rasterizing dialog. Now the color picker gives the color, no calculations are needed.