set the colored layer to multiply

the line art are to thick if you using the vector layer or some sai version say the linework layer

after adding line and curve use the weight tool to re size the line foe example
if you use the line layer as 1 pixel then use the weight layer as 2 or 3 pixel

after that use the pressure tool to adjust the line at curve point so it look like it was sketch from pen

enhance it or reduce the layer thickness using this pressure tool

i ll try to work with your art and show you how it look like soon

there are  many  style
1 is like your friend say
those are color over

but you need to understand the concept first

and that is layers

with your friend style the line art it at top

other style like mine is line art bottom
try this

line art layer bottom  layer = normal
colour layer top    layer =  multiply

haiya surely you have a save file

open that save file and take the lineart out

dont tell me you colour on the line art

this is easy girl here i finish tracing it

like this

then .... you cant

if its jpeg its a finish product

wait i upload another 1 for you

problem is i dun even can see your shading, sometimes just make it more contrast, no need scare one 

hmm..but u can control the oppacity u want right? dont worry its digital afterall..you can adjust later