I think the correct parameter is border_color
whose default is "grey60", so I suspected (incorrectly as it turned out) you were getting partial name match from your argument "border" whose value becomes colors()[1]
which is [1] "white".
Try:
library(pheatmap)
pheatmap(tissuedata3,
color = colorRampPalette(rev(c("#D73027", "#FC8D59", "#FEE090", "#FFFFBF", "#E0F3F8", "#91BFDB", "#4575B4")))(100),
cellwidth = 20, cellheight = 3, # changed to 3
border_color="blue",
treeheight_row=0,
treeheight_column=0,
kmeans_k = NA,
show_rownames = T, show_colnames = T,
fontsize=2,
scale="none",
clustering_method = "complete",
cluster_rows = FALSE, cluster_cols = TRUE,
clustering_distance_rows = "euclidean",
clustering_distance_cols = "euclidean",
legend=TRUE
)