You may well use a computer program to create the colors, only stay diciplined. Use some better tools than setting random RGB or HSV numbers. If you search a well ordered RGB list for some real and known pastel color material set, forget it. The RGB numbers depend on light, paper, how the color is rubbed to the paper and how the result is photographed to get a RGB image.
Get GIMP (=freeware). It's LCH color selector for RGB color mode images can give well ordered and consistent results. LCH is like the CIELab color system, but instead two coordinates a and b, a polar system with chroma and hue is used for more intuitive variation making. L is the same both in LCH and CIELab. I guess it's just what you expect
From the web you can find easily color lists which are said to be pastel color schemes for design. This is one example:
Some of them (like this) can be well chosen sets to make a certain design and totally useless for many others. The pastelliness in these colors is
- not especially dark, generally the lightness is more than 50% of the maximum
- low chroma (=chromaticity, the visual colorfulness, subjective difference from the grayscale)
Low chroma is not the same as low saturation, because saturation is the relative chroma, the percentage of the max possible chroma at given lightness level in RGB system. A low chroma color can still have 100% saturation when the color is dark or bright. RGB system can generate maximum chroma at near 50% of the max lightness.
Here's a screenshot of GIMP's color selector dialog:
It is set to show the colors at the same time as RGB numbers and LCH (=lightness, chroma, hue) numbers and all of them have sliders.
The purple areas in L,C and H sliders are the areas which are out of the RGB color range, they would need RGB numbers beyond the range 0....100%. (100% means 255 or hex FF)
For example if you set quite high chroma, only a zone in medium lightness range is allowed and also some hue values can be unreachable:
This red #E95171 in RGB hex has limited lightness adjustment range and limited hue adjustment range due the limitations of normal sRGB displays.
If you search pastel looking colors for RGB images, have guite high lightness, more than 50% and keep the chroma at so far left that the whole hue range is available:
Here lightness is about 75% of the maximum and chroma is 28%
Drag the set color to the quick palette in the bottom of the dialog or paint a dot with it or copy and paste the hex RGB numbers to somewhere.
By changing hue you get different visually as colorful and as bright colors. By changing lightness you get as colorful brightness variants of the set hue as far as the RGB display allows.
At last in this phase one should ask "why LCH? HSV system should give the same!" It's not the same. In HSV the subjective chroma varies when one adjusts hue. In LCH the parameters are tried to make subjectively independent. I must admit that in low chroma colors such as the pastel looking ones, HSV can be good enough.
How to make a harmonic collection for some work? This is different question. It's beyond the scope of this answer. But if you already have one, then moving the hues as much can create an usable variant.