Thanks for the bug report. This should be fixed for the next release.
This issue has been around for a very long time, maybe from the beginning. It also affected every color option (not just SolidColor). The issue is tricky since Rainmeter parses things differently depending on if the "option exists" versus the "option exists, but is empty". To the user, both of those should result in the same "default" setting. But in this case, colors are parsed a little differently mainly because each color channel is defaulted to 255. I believe the original thought was each color channel needed to be "something" rather than "nothing". Especially for the alpha color channel. If that defaulted to 0, no color would show at all.
Anyway, the fix was simple. Before going to the color parser, we simply check if the option is empty and return the default value.