Hi Moya, that looks like a wonderful project!!
What I do, if I am not sure about colours, is I make copies from the pencil sketch and colour them to test colours (when it's a more complex project – I don't do that with an envelope
! I sometimes make a colour scheme (just touching squares of colour) and try out different combinations. For example if I know, I'll need one dark colour for contoures and 3 to 4 lighter shades, I'll draw a square, divide it in how many colours I want to use, and fill them with different combinations. I use magic markers (because I have them still from my study days, I wouldn't invest in them just for that), you can use anything – crayons, markers, watercolours. Then I test it in the copied layouts. Then I'd mix the colours finally (or finish the piece in Photoshop).
For inspirations you can use anything – sometimes I take a photo, a painting, a poster, a look out of the window that fits the subject of the project … (for example like you say, a card for a newborn boy. Look out the window, blue sky, light brown houses, a red car – so that's a colour scheme I can work with, I have two background colours, a signal colour, maybe I need to add a colour for depth / contour, maybe dark blue …)
You see, my process is very intuitive. Usually if there is a client I still can explain quite well why I chose the colours, i.e. why the colours are just right for the project
.
You can also think in terms of should this be controversial (complementary colors), funny (colourful), sad (less saturated colours, colder hues), futuristic (you could use neon colours + metallic tones) and so on …
(I do this now often in Photoshop, but I am really fast in Photoshop, so I also wouldn't recommend it if it's not an everyday tool for you
. This is just meant to be a fast way to try out your design and make variations.)
I hope that helps!