Should be 2 squares overlapping each other with a little arrow next to them. Click the top one to change colour of your main brush, click the second to change the colour of your second brush and use the arrow to switch between. This is providing you have the toolbox visible. Click Ctrl+B if you dont have the toolbox.
I haven't actually used it myself since I have Adobe Premiere Pro, but even some PP users are considering moving to Resolve at the moment, I gather. It's a professional NLE and colour grading system, so may be a bit over the top if you are just wanting to do basic stuff.
There is also Blackmagic Fusion (https://www.blackmagicdesign.com/products/fusion) for visual effects and animation type stuff that some feel is better than Adobe After Effects. This, of course, has an even higher learning curve, but is very powerful. If I remember rightly, the fully version was used on films like Avatar. Again I have't actually got round to using it myself, but I have watched a few tutorials and demos.
I always use the Corel VideoStudio, it's version 10 or 12, I don't remember because it's on my other PC, I suppose it's free ?
Anyway, it's real good for greenhand like me, it has special effect and many other useful functions, the most important is the interface, very easy to operate. The only problem is sometime crashed, so always saving the project while you editing! Other video editing software, I think the window movie maker and imovie are cool for new learners.
Flowblade, Kdenlive,Openshot,Avidemux are all free open-source video-editors, but I use them on the Linux platform.
Gimp , Krita, Lightzone, Darktable are all open-source photo-editors.
Audacity, ( in my opinion ) is the best audio-editor for Linux.
All of these have " How To " videos, many Gimp videos are available on the Gimp website, and also Youtube.
Audacity, etc, also have videos on Youtube.
Here are a couple of examples.