How do you know what values to enter to make a color? Is this just a matter of dragging the sliders?
With RGB the higher the value, the lighter the image. Anything with three values over 240 will always be a pastel color, and over about 250 you are getting to off whites.
If only one value is high, then you will get that color. 255/0/0 is pure red. If you lower the value it gets darker 128/0/0 is a dark red, and when you approach 0/0/0 it gets near black.
To make a lighter red (pink) increase the other two values. So 255/128/128 is a pink. The higher the other two values, the closer you are to white, and the lighter the pink.
There are complementary colors CMY (no K here). To get Yellow, you want the reverse of Blue. Blue is 0/0/255, so Yellow is 255/255/0. The other complements are Cyan and Magenta.
AS you say, playing with the sliders can be informational, but these basics will help.