In case you don’t already know, Redbubble recommend that you produce/design your artwork using in CMYK format and then convert it to the sRGB profile before uploading for use in Redbubble. Notice that they don’t say which CMYK profile - sighs.
Here is their article about it:
Designing in CMYK vs RGB
This is not a good thing to do in general but I can understand why they give this advice.
It’s to reduce the number of complaints from disgruntled and inexperienced artists who would be disappointed and annoyed that their printed artwork didn’t look the same as it did on their PC screen if they designed it in RGB.
Working in CMYK has many disadvantages but it may be just about ok for simple painting artwork. Some people here would tell you that working in CMYK is always a bad thing to do. However, there is a better way.
You should work in sRGB and use the softproofing facility View → Softproofing (Ctrl+Y) to see what your artwork would/may look like when printed and you can use the Out of Gamut Warnings (Ctrl+Shift+Y) to show you if the artwork colours will get ‘crunched’ when printed with a CMYK process.
Note: @Takiro may tell you that this is all dependent on your monitor being properly calibrated in the first place, which is hardly ever the case 