Well depending on how you made the child theme. and what you did when you used the decaf child themes, you can add the CSS changes in the child theme's Raw SCSS boxes. The first "Raw Initial SCSS" box to change the variables that govern the primary and secondary colours. Then, depending on what colours you want to use in your themes you could create a set of variable like I did for the Moodlecandy preset, which was to use the Brand-primarhy colour and then assign other variables, such as link, visited, hover, active and focus, heading, etc using lighten($brand-primary, 5%) or darken($brand-primary, 5%). by whatever percentage you feel is the color you want for your themes.
The beauty of this is you would only need to change the Brand-Primary colour to the color that that child theme was alloted. so Blue for the 'bluetheme' , red for the 'redtheme', yellow for the ;yellowtheme' etc.
To see where I am going with this you could try installing the Morecandy Preset into your child theme and see how that works. If you liked that idea then all the changes you would need to the child theme is set the Brand-Primary colour to the main colour scheme for the Child theme.
Hope this helps, but please do feel free to ask more questions, as it can seem a bit daunting at first.
Cheers
Mary