Right now, the tier-color scheme in Factorio is all over the place: belts go yellow->red->blue, electric circuits go green->red->blue,
assemblers go gray->blue->yellow, inserters go gray->yellow->blue->green (furnaces are 3 completely different-looking entities, while electric furnace is also ...

just as there is a small battery icon in the bottom right that indicates the amount of battery in the armor the engineer wares, please add another, similar icon to indicate the amount of charge in the spidertron that the engineer drives.