Well, practice. I use many mods by a modder who writes into every readme file "proudly made with paint" - so it obviously is not a matter of the tools but on their use... Check other author's graphic files and learn their techniques.

One thing that I've found useful when making top down type mods (although not for NIMBY) is to basically paint new layers over the top of a top down view of that vehicle downloaded off the internet.

I use Paint.NET for mine. Gradients are the key to looking more realistic. And making lighter colours darker than you think they are. For example, having your "white" be not 100% white, allows to have highlights. I achieve this lazily by use of a gradient layer that blends into the colour layer.

I use 4 layers to make a train. From top to bottom they are:

Outline - Used to draw initial outline of train. Not exported in final product.
Fittings - Things like handrails, grilles etc
Gradient - Shading to add contours and texture to model. Blends down with layer below.
Background - The colours. So far, all the things I've made are real world colour scheme, so I haven't had much experience with user-coloured items in-game.

Will upload some pics in a new comment.

Here are the pics of the process I mentioned above:

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289571942

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289572740

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289573367

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289574221

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289574645

引用自 trolley_trev：

Here are the pics of the process I mentioned above:

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289571942

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289572740

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289573367

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289574221

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3289574645

This is super detailed and helpful thank you!

引用自 trolley_trev：

I use Paint.NET for mine. Gradients are the key to looking more realistic. And making lighter colours darker than you think they are. For example, having your "white" be not 100% white, allows to have highlights. I achieve this lazily by use of a gradient layer that blends into the colour layer.

I use 4 layers to make a train. From top to bottom they are:

Outline - Used to draw initial outline of train. Not exported in final product.
Fittings - Things like handrails, grilles etc
Gradient - Shading to add contours and texture to model. Blends down with layer below.
Background - The colours. So far, all the things I've made are real world colour scheme, so I haven't had much experience with user-coloured items in-game.

Will upload some pics in a new comment.

Where do you find exact measurements and the outline? Is there a way to convert from photographs?

引用自 adlet：

Where do you find exact measurements and the outline? Is there a way to convert from photographs?

This is the difficult part. So far, I have been able to get a hold of technical drawings for almost everything I have made. The rest I made some educated guesses. I then hand draw the outline and things via doing the math. :(

Photographs could work but it would be hard to get a good top down view with enough detail. Possibly Google Maps if you can find an example of a train you want to make. But once you do, it would then be a matter of scaling to correct size and then hand drawing the outline on another layer over the top.