Shading in hero web part different in upper and lower row

I am using the Hero web part with five tiles. All tiles have an image with a blue background, all same even blue 0x004370.

Still, the shading of the four small tiles is not the same, the two in the upper row is darker than the two below. For clarity I have set all images equal in this example:

Can someone tell me if I am doing something wrong here - or is SharePoint just beyond control?