The free and open source vector image editor Inkscape might work for you, however it's not particularly easy software and there is a learning curve, but Illustrator is similar in that respect.
In Inkscape a little setting up would be required. In the example below I used an SVG that I downloaded. The pattern itself is on one layer which I locked in the layers panel. Then I made a new layer for the fills underneath. Then I used the Fill Bounded Areas tool (aka Bucket Fill tool) to fill it with colours.
You could also unlock the pattern layer, select shapes and change the stroke colours as required in the Fill and Stroke panel. You can also apply fill colours directly in this way.
Other possibilities include free and open source raster image editors such as GIMP or Krita. Both have a Bucket Fill tool that can be used similarly. Both GIMP and Krita can open SVGs, but they will be rasterized. Use Inkscape if you want keep everything vector. Inkscape is like Illustrator. GIMP and Krita are more like Photoshop.
Note: I have no affiliation with any of the software listed above. These are merely suggestions. All three I have suggested have fairly large user communities, plus a plethora of tutorials available on youtube if you need to learn more.
