Changing certain areas of color before printing

[stephen hammond]

Firstly, I am a total novice with sort of stuff so be kind....

What  I need help with is as follows

I construct card buildings which are printed on to paper before being fixed to card and cut out. Each sheet has certain parts of the building, walls, roofs, doors etc
What I need help with is I want to just change the colours of the doors for example which are a blue colour. A friend has done this by getting the page on screen selecting the area to change, by just clicking on the door colour, and selecting a new colour, so nothing changes except the area chosen
This all seems easy but not to me!!

Help needed and if possible in simple terms

Thanks Steve

[rich2005]

Always better if you can post an example image but in simplest terms

The color select tool, clicked in a color can make a selection. Adjust the threshold and re-click in the color until you get a selection that covers the required area. 

   

Then for simplicity I would use the Colorize filter  menu Colors -> Colorize and adjust the sliders until you get the required color change.  Ok everything and kill the selection Select -> None

   

As with everything there are snags making a color selection. Often picks up other parts of the image. There are ways of limiting the selection to just the door.