Help with color selection for interiors

Simply make the solid color lighter? Or what is meant by that?

I suspect a causal connection between your lack of hunting success for the desired pastel shade and the search for something that does not exist. For paints you have to proceed like this: mix the pigments directly in the target color, and then there are color charts for the ready-made shades. However, for painter's paints, it is done differently, precisely because of people like you who find a pastel not delicate enough: here you take white with a dash of color. There are color charts for this too, but more rarely: namely those where the tint number XYZ is displayed quasi "tabularly" as it appears in mixtures with 2, 3, 4 or more parts white to 1 part color. On the PC you can simulate this—careful not to forget calibration effects!— by copying a color patch representation (there are also various "translation tables" RAL to RGB online) into a graphics program (e.g. Open Office Draw), then placing a white layer over it; then you lower the transparency of the white layer. That way you should find a suitable solid color which you can then have the painter mix with the corresponding multiple amount of white paint. You will hardly find a ready-made pastel painter's paint because, due to the variety of tastes, it is decidedly more economical to always mix pastel from white and color.

Have also found a color that we basically like, but it is too dark on the wall. I have considered lightening it with white. Problem in my eyes: you can't get this mixture a second time. So if I have to do some touch-ups, repaint, or in the worst case, if the paint is not enough, I would have to paint everything new. Therefore, we are now first trying with a hardware store fan deck, because these colors can be bought again.

Problem in my eyes: You can't get this mixture a second time.

That's why you don't mix "randomly," but in a ratio you can remember, usually "whole number : one." The method with tinting colors clearly has its domain in pastel tones, i.e., where conversely the color pigment has the greater "weight" in the mixing ratio, it usually is not convincingly suitable. But the OP is looking for a tone within the domain of the tinting method.

Update on the topic: I now have a color fan from the painter or rather from his wholesaler. The desired tones are definitely included, because it's exactly as 11ant says, there is a base tone and then the shades get lighter in a table format, with precise designation. My painter uses Caparol. I can now insert the Caparol colors that interest me into our Sweet Home 3D model on the PC, see how everything looks together, and also see the color tone in real life (in different lighting) on the fan. I think this should work out. Thank you for your help!

With Caparol you have a good product, TOP quality, not a mediocre product. We have already purchased and used this from our painter several times.

We just had this topic yesterday with our painter...
He said, nowadays no one makes a frame anymore?!?
And he works at a painting company that already painted the "more modern" buildings in the city (restaurants, department stores, architect houses etc.)...

I hate it when service providers/craftsmen come around with "that's not done anymore." It's MY house and I truly don't care whether something is trendy or not anymore ...... I wouldn't let that stop me unless they have arguments why it's no longer done, because a pro/con XY convinces me.

We also found a color that basically pleases us, but it's too dark on the wall. I considered lightening it with white. Problem in my eyes: you can't get this mixture a second time. So if I have to do touch-ups, repaint, or in the worst case, if the color isn’t enough, I would have to repaint everything. That's why we're trying it first with a hardware store fan deck, because those colors can be repurchased.

If it's mixed according to a formula, then you can always buy it again. But mixing it yourself "by hand/roughly" really won't work.